Just because the game designers made a good game doesn't mean you can't make it even better. Or at least different.
Game modifications, or "mods" for short, are any alterations to a game that were not made by the game's license holder. They can be unofficial Expansion Packs (new maps or new equipment in the same game), completely unrelated games that merely use the source game's software as a backbone ("total conversions"), or just quality-of-life adjustments to the original, such as Fan Translations, bug fixes, character Cameo appearances (which can often lead to Ninja Pirate Zombie Robot scenarios), or House Rules. (Or, if the game includes any attractive women or men, nude patches.)
Some games are deliberately designed to be easy to modify, including a "construction set" of sorts to build levels, weapons, etc., and whole modding communities spring up as a result. However, some types of mods are discouraged, or even cracked down on, for good reason: a mod in the hands of a player but not his opponent usually means an unfair advantage. (Some God Modders will use these anyway and hope they aren't caught.)
A few mods take advantage of content that was programmed into the game in the early stages, then scrapped from the final design. Unless space is a big issue, the programmers usually leave all this content Dummied Out in the game's code. This leaves an opening for a modder to re-introduce an access point and enjoy the missing content... albeit at their own risk, since it's usually unfinished and untested. A safer and more success-guaranteed approach is the graphic overhauling. PC hardwares advance through times, making it capable of handling more detailed graphic and it is required for an old game to look new and fresh to keep it alive among the community, and to avoid making the game being overwhelmed by current-gen games.
It's much easier to mod a computer game than a hard-coded console game, but creative adjustments to a save file (and, with the most recent generation of games, console hard drive content) and/or use of a Game Shark allow determined amateur programmers to mod with the best of them. Most often, however, mods of console games are edits of the ROM files used in Emulation. These tend to be referred to as "ROM hacks" and are usually considered a separate thing from mods, even though when you get right down to it, they're really the same.
Early ROM hacks were largely present in Famiclone consoles and cartridges, especially where the said system wasn't officially released. Cartridges contained graphics hacks very often. The most common ones were those where the main character was replaced by Mario.
Unfortunately, ROM hacks are probably the biggest offender of Sturgeon's Law. A few years ago, a great many ROM hacks consisted of little else but tons of offensive and poorly drawn graphic hacks like profanity-laden graffiti, buckets of Gorn, nudity and outright pornography, and (just to offend anybody who wasn't offended yet) swastikas, racist slurs, and pro-KKK propaganda. Most modding communities have very little moderation or quality control, save for a few that went on Quality by Popular Vote. In addition, redundancy is a bit of a problem as well - a lot of mods might accomplish similar things.
As mentioned earlier, computer games are much easier to modify than console games. As such, PC gamers will often list mods as a reason why PC games are clearly superior to their console counterparts, despite the aforementioned Sturgeon's Law
See also House Rules and Self-Imposed Challenge.
Some of these even have their own work pages on this Wiki; see Game Mod Index.
Here are examples of the popular modded games among the modding community. Feel free to add in a game and some description if its community endorses it enough:
Which itself has its own devoted mod community, making game modes (like the ever popular space build), scripted items (lua), new models, scripted weapons (sweps), new tools (stools), new maps, textures, materials, and scripts.
Just about every mod for Gmod requires Wiremod, plus then there are the 4 Stargate addon packs, the Stargate addon pack by pyrospirit, Mckay's Addon Pack, Carter's Addon Pack, and SG-Mod, and then there's the SBEP for Spacebuild.
Mods are Valve's bread-and-butter. Team Fortress began life as a free mod for Quake, but Valve hired the staff, recreated the mod for Half-Life (calling it Team Fortress Classic) and then created their own sequel, Team Fortress 2. Similarly, the Reaper Bot was a Quake mod which created AI players to fill out smaller deathmatch games. Valve hired that guy to help with the AI soldiers for Half-Life, and it became one of the huge selling points of the game. Ditto Day of Defeat and Recoil, mods purchased by Valve and integrated into their releases.
Valve's Alien Swarm is also based off an Unreal Tournament 2004 mod and the inspiration for Left 4 Dead was the popular zombie mods for Counter-Strike.
Speaking of Team Fortress 2, there are fans out there who love to make texture hacks to change the appearance of weapons, hats, character models, etc. to further hilarify their game experience. FPS Banana has a ton of skins for your perusal.
Why go to a site when the game now accepts custom content as actual content? This part of the main site (Contribute!) is where anyone can send their custom models, letting Valve put them in the game. As of now, there are 88 community made Nice Hats and 29 weapons, all the visuals sorted out by talented members of the community.
There are also quite a few fun game mods out there: Pyro Dodgeball, Prop Hunt, Vs. Saxton Hale, TF2 Ware, and Zombie Fortress, to name a few.
Vs. Saxton Hale has also clearly inspired the first Halloween Boss, the Headless Horseless Horseman.
There's also mods that alters the gameplay of existing modes. Like Randomizer, respawn time disable/reversal, class limitation and it's related Highlander mode, and Hardcore- game servers with crits disabled.
Anyone play Adam Foster's MINERVA mod and noticed the excellent and atypical style of level design he used? Valve did, first helping him out with the last couple of chapters, and then hiring him to work on HL2: Episode 3!
And part of MINERVA's inspiration was Marathon. Fuck yeah.
The Iron Grip: The Oppression mod for Half-Life2, which slammed two genres against each other: RTS and FPS.
Empires combines RTS and FPS like Iron Grip, but in a different manner; players play on one of two different armies fighting each other, each with their own commander to whom the game functions like a standard RTS.
On top of the typical skins and script modifications, the Left 4 Dead games have a level editor that allows people to create their own survival and campaign/versus maps. They can even include custom sounds and music, scripted crescendo events, and other things. Naturally, there have been maps of other games like Mario, Zelda, and even Half-Life 2. Then there are the Mutations, "official" modifications that come in packed with the game. Recently the developers have encouraged players to develop their own Mutations for submission to be featured for a week. They've also been featuring user-created campaigns on the official blog, and hosting them on the official servers.
Portal's honorable mention is Portal Prelude, an unofficial prequel that introduces fairly-harder-than-original levels, new puzzle elements and several additional hours of gameplay for hardcore Portal fans, which got one million (!!!) downloads and a following of its own.note Though, some might argue that this isn't strictly a mod, in the way that it doesn't change Source's source code, but it is worth playing nevertheless.
One of the earliest game series to still have an active modding community today is the Doom series, which intentionally allowed modding of all kinds (Wolfenstein 3D wasn't designed this way, but fans were able to mod it anyway), with the first editors coming out just months after its release. One particular Mod, The New Technology: Evilution (later known simply as TNT: Evilution,) was intended to be freeware, until it was announced at the Usenet Doom boards, on its intended release date, that not only would its release be postponed, but it would become a commercial product—published by none other than id Software themselves! The reaction to this announcement was... heated, to say the least. The product would later come out as part of Final Doom—within the same month as Quake. And the only place where the team was credited—the DOS text-mode post-exit screen—didn't appear in the bundled Windows 95 version, which is how most people installed the game anyway.
And due to using the Doom engine, the games Heretic, Hexen, and Strife also have a decent number of mods for them, though not as many as Doom. In addition, there are also source ports designed to be compatible with multiple Doom engine games, allowing the creation of mods making use of the various features introduced in those later games.
On a lesser scale, Doom 3 has a large amount of mods available, which range from solving common complaints (adding flashlights to the weapons [Finally, there's Duct Tape on Mars!] and correcting the dismal mechanics of the shotgun, for example) to full stand-alone singleplayer campaigns.
One particularly notable mod for Doom 3 is darkmod, which is a package to convert Doom 3 into something suitable for building Thief-like game content in. This is partly due to the Thief mod community not generally liking the Unreal engine based Thief Deadly Shadows for modding (there are some strange differences due to changing engines, and it turns out to be very hard to get things that should be a doodle, like a passageway you can just walk down, to work properly).
Into Cerberon is a total conversion into a Descent fangame. The name's based onDescent Into Cerberon, a music track from Quake II.
Duke Nukem 3D is another game that has had a fairly large number of game mods and TCs created for it. A list of the more recent mods is as follows.
Duke Plus, created by Dan "Deeperthought" Gaskill, started as a means to enhance the use of level effects in user maps by adding elements such as realistic water that could have both above-water and under-water effects in the same sector, in addition to have the ability to mirror and expand the size of existing maps for changing the experience of existing levels. It also possesses weather effects like rain and snow, a realistic acceleration system for running/moving, the ability to grapple along the sides of walls [i.e. mantling], improved enemy behavior, the ability to pick up, drag, or throw various items like crates or other background objects, etc. Some new weapon types were also later added, including a laser pistol taken from the Assault Captains (with a chargeable fire function), a gravity gun substitute to the Expander, two different types of shotguns instead of the original, among other things.
Naferias Reign Invasion Of The Dark Mistress, a mod created by Jack "Lord Misfit" Walker, is still in the works, but many betas are released, often once every two or three months. This mod might be considered to be one of the biggest FPS/RPG hybrid mods for a game yet, as it not only incorporates normal RPG elements like leveling up during gameplay, but allows use of a group of 15 different characters, including Duke Nukem himself. The kicker: this isn't like Hexen, where a character is chosen for the whole of the game, but where characters are found over the course of the game and can be swapped back and forth via an extra menu system. Each character has their own skills, strengths and weaknesses and their own main functions in the team. And that's just a few of the numerous features of the mod. Listing them all would take up too much space.
Desert Combat for Battlefield 1942. It has the same objectives as the original but transplants the setting from World War II to Operation Desert Storm, updating the maps and weapons accordingly.
Interestingly, much like Valve has a habit of hiring good modders, EA bought Trauma Studios (makers of Desert Combat) to work on its modern-day sequel, Battlefield 2.
The original Battlefield 1942 had several total conversion mods (Desert Combat being the most popular). There was Forgotten Hope, a realism mod adding a large amount of new content and balance changes to give a more gritty, realistic WWII; Eve of Destruction, a total conversion mod set in Vietnam; and Silent Heroes, a total conversion mod based on a speculative conflict between Sweden and Norway, just to name a few.
A mod team was also working on a mod in this engine, but limitations forced them to move it to Source (see "Empires" above).
Battlefield Vietnam had an official World War II mod, which basically turned it into an updated version of Battlefield 1942.
It's hard to mention Battlefield: 1942 mods and not think of the Turbo Mod. The speed of all weapons and vehicles was absurdly increased and alternate fire on all vehicles activated a rocket boost. Multiplayer games with this tended to devolve into goofy stunts like a never-ending loop-the-loop with jeeps or jumping over Midway island using a battleship.
There was also a Star Wars-themed mod once, initially with only one map based on the Battle of Hoth but massively expanded over time to include a huge variety of locations. Apparently, this mod was the inspiration behind the Star Wars Battlefront series.
There is also Project Reality, a mod for Battlefield 2 that aims to make the game more realistic. It is so popular that some people buy the game just for it.
The "accessible realism", WWII-themed Red Orchestra would be a good example. It was created for Unreal Tournament 2004 (an odd choice for a realism mod), and after much effort, went on to win $1,000,000 and free Unreal Engine 2 licenses in a Mod Competition (nVidia's "Make Something Unreal" contest). The team, Tripwire Interactive, has since released a retail, stand-alone edition of their mod, entitled Red Orchestra: Ostfront 41-45, eventually followed by a sequel on Unreal Engine 3.
Tripwire has since also published Killing Floor, another UT 2004 mod gone retail, as well as having created Red Orchestra 2: Heroes of Stalingrad on Unreal Engine 3.
Unreal Tournament in general has lots of mods. One of the most well-known ones is ChaosUT, an offshoot to the original mod released for Quake II. It adds a complete arsenal of guns, some of them with multiple kinds of ammunition (normal, armor-piercing, poisonous and/or explosive); Ur Example is the Lethal Joke Item known as the Proxy, a sentient claymore which chases you down with a smiley face and a cute voice. Or the new superweapon, the Gravity Vortex, which creates a micro-black hole that sucks in and gibs everyone in a substantial radius for several seconds; it even ignores the godmode cheatnote it does so because it uses the crush death coding and crushing in the unreal engine 1 bypasses godmode. Aside from the guns, other accessories were added too like Gravity Belts as toggleable jumpboots and grappling hooks to move over impassable terrain. And who could forget the ability to amass your remaining rocket ammo and go kamikaze... Oh, and the bots are freakishly good with all weapons, especially the crossbow.
Then came ChaosUT2 for the sequel, bringing with it a graphical upgrade and a few new additions like the Chaos Pulse Pistol as dual-wieldable spawning weapons, the C.U.T.T.E.R. which is a badass-looking version of the Ripper, the Chaos Grenade Launcher with four types of munitions (explosive, poison, napalm, flashbang/EMP) and the Vortex Launcher which now enables the user to actually run away from the black hole before it activates. The MUG (Multi Utility Gun) now operates like a normal flamethrower and the Claw can be loaded with needle packs whose projectiles stick onto whoever touches them and continually deals damage. It even added two vehicles: the easily forgettable hoverboard and the Chaos Bike.
Then came the sad day when the dev team announced that due to the lack of interest in UT3, ChaosUT3 was cancelled.
Oldskool is another notable modification pack. It allows true single player campaign gaming, supporting the original Unreal as well as several custom map packs (Operation: Na Pali and Nali Chronicles being two of the most famous). It also works as a customizable mutator for practice sessions and multiplayer.
Speaking of UT2004, there's the Ballistic Weapons mod that converts the game into a realistic tactical shooter complete with a huge arsenal of guns and features like dynamic accuracy and recoil; toggleable scopes and silencers on many of the weapons; toggleable firing modes between single-shot, burst-of-threenote one submachine gun has both 3-round and 5-round burst and full auto (the minigun instead has single-shot, 1800RPM, 2400RPM and 3600RPM); the loadout system which lets players customize their spawn inventory... All but two guns are kineticnote three melee weapons, seven pistols, three submachine guns, four shotguns, three assault rifles, battle rifle, two heavy machine guns, minigun, sniper rifle, sniper railgun, rocket launcher, flamethrower, Lightning Gun, three grenades, remote-controlled explosive, land mine and are provided with a backstory. Recoil is very realistic: it's impossible to fire the minigun at full speed and hit someone unless the built-in bipod is deployed first (machine guns have bipods too). All pistols and submachine guns can be dual-wielded, even different ones.
The sheer number of mods released for Starsiege: Tribes and Tribes 2 (both of which are now freeware by Sierra releasing and abandoning it) is fairly astounding, not to mention what's been done with the vehicle systems.
Fans have produced countless third-party maps and mods for all three Marathon games, including a number of "total conversions" (eg: Devil in a Blue Dress, EVIL, Erodrome, Excalibur: Morgana's Revenge, Portal of Sigma, RED, Rubicon, Tempus Irae, Trojan.) The third game, Infinity, was thus named because its headline feature was a set of polished 1st-party modding tools.
Both Thief: The Dark Project and its sequel, Thief 2: The Metal Age have loads upon loads of new items, UI improvements, AI, missions, and even entire campaigns! Though the first game is over ten years old, the mod community is still plenty healthy.
Thief: Deadly Shadows used a variant of the Unreal engine, and was generally panned for mod purposes (besides weirdness like Garret now having Super Drowning Skills, it turns out to be very hard to get Garret to not walk drunkenly, and other issues). This helped inspire DarkMod, a fan-created conversion kit for Doom 3 (a game often panned for just not being very good) which makes it suitable for Thief-style games.
The long period after Looking Glass Studios folded and it looked like there would never be another Thief game led to the fan-made Thief 2X: Shadows of the Metal Age conversion/expansion of Thief 2.
Halo: Combat Evolved has an extensive map modding community, which often range from crappy to excellent. All of which is played on a separate version of the game called "Halo: Custom Edition" (which requires the retail version to work).
Deus Ex still has a very strong modding community despite being an older game. This includes graphical and gameplay tweaks such as Shifter, and many total conversions. The most impressive of these is The Nameless Mod which took seven years to complete and is a complete game including Deus Ex gameplay, branching storylines, and thousands of lines of voiced dialogue.
Zodiac is another one, a fan made Interquel that allows you to play as Paul Denton.
2027 is a fan-made prequel featuring a nonlinear plot, real-world weapons, and DX9 graphics.
STALKER also has a number of mods, including the Complete 2009 mod, which provides a massive graphical upgrade and a wide range of gameplay tweaks.
AMK: Narodnaya Soljanka for Shadow of Chernobyl may not be as much of an eye candy, but more than makes up for it by bringing in lots of levels from Clear Sky and Call of Pripyat, as well as a mind-boggling variety of new gear, new artifacts, consumables and a ton of new quests. Needless to say, the game world gets huge enough to make a complete playthrough qualify as a virtual vacation of sorts.
Several of these have been made for the Descent engine, such as Pumo Mines(a total conversion) and Descent Vignettes.
The modding community for the games of the Dark Forces Saga was also very thriving in the past, especially for Dark Forces (link directs to the major DF fansite DF-21.net) and Jedi Knight (link directs to the major JK fansite The Massassi Temple). And even so, new levels are still being worked on even today, with recent major releases including Magrucko Daines and TODOA. Some mod developers, such as Patrick Haslow (creator of The Dark Tide) have even worked on major titles such as Wolfenstein and Bioshock Infinite.
The Wolfenstein 3D community may not be as prolific as Doom's, but there are still many, many mapsets and conversions available for the game.
The infamous Hot Coffee mod for Grand Theft Auto: San Andreas. In the game, CJ can have sex with various women, but the depiction is limited to seeing them entering the woman's house. Hot Coffee re-enables a Dummied Out sequence where the player not only gets to watch but controls the action to a degree. It was later patched. The discovery led to a brief revival of the Games Are Evil panic.
Incidentally, PC players tend to avoid said patch not because they want to enable it but rather because it interferes with actual mods.
Speaking of which, the PC release of San Andreas has considerable popularity in Japan because of or leading to the mass number of Touhou mods made for it. I know you're thinking that Marisa in a world full of gangsters does not compute but still...
The Grand Theft Auto series of games has always appeared to be ripe for multiplayer, but it was never implemented by the designers. Enter Multi Theft Auto, a mod that allowed for the player of the PC version to play multiplayer with large numbers of players and its own custom game modes. Multi Theft Auto was released for GTA III, Vice City, and San Andreas, and is massively popular with the PC players for all of them. Rockstar got the hint with the PSP version of Liberty City Stories, which had online multiplayer for up to six players. They completely polished it for GTA IV, with 16-player (32 for the PC) support and 15 game modes. GTA IV's multiplayer has some similarities to MTA, including customizable character models, the whole city to explore and fight in, and some game modes.
The Escape Velocity series featured the ability to mod the games simply by dropping the mod (or plug) into the Plug-ins folder, much like the Apple Macintosh's operating system could be modified by dropping new extensions/control panels into the System Folder. The most complex mods, however, require overwrite of key data files; ironically, the total conversion mods for the third game that allow you to play the first two games (mods that were released by the publisher) fall into this category.
For the most part, it was not so much that it required overwriting as that it was easier to design total conversions to require you to switch out the core data files for the TC's data files than to have loads and loads of empty resources to keep things from the original game from showing up when they shouldn't.
Being an ASCII-based nongraphical game, Dwarf Fortress is quite friendly to modders. Some of the more common changes to the game include new wildlife, plantlife, and races, the ability to play as non-dwarven races, and changes to existing animal stats. Like fire-breathing bears.
It's also been modded into Ancient Roman Fortress and Fallout Fortress. There are both "beginners' versions" that are less sadistically difficult that turn your dorfs into little Terminators and versions that make every monster six times stronger. There's also some really dreadful cheating possibilities in playing with the raw files; it's possible to make your forges spit out Unobtanium, for instance, allowing your tiny outpost to fleece an Elven caravan out of everything they own in exchange for one earring.
Notably, the author of the game has said he wants the game to be as mod-friendly as possible. An example is that while he doesn't plan to have explosives in the "vanilla" game, once it's finished, he wants modders to be able to add this with just a tiny bit of code.
Minecraft, having been open to modders since the beginning, has a very large modding community with lots of mods to add on. Some add simple things like improved crafting recipes, others add entire new worlds.
A community under the name of Bukkit has numerous plugins used on multiplayer servers (well, on the easier to manage Bukkit servers) with a variety of features. Some are for anti griefing measures, such as disabling damage to terrain by explosions and disabling fire from spreading, to having an in game economy for role playing purposes. The Bukkit community has gained so much attention and popularity that the developers of Minecraft have plans to incorporate the game with the Bukkit plugins to make modding easier. The plugins can be found here.
There are also modpacks (multiple mods bundled into one) such as feed the beast, yogbox, and so on.
Baldur's Gate II, even well after a decade past its initial release, still boasts a very active modding community. Most common additions allow the player to complete new quests, recruit new NPCs, and add new dialogue and romance options. More technically, there are mods for new items, shops, character class kits, and adjusted combat and alignment mechanics. Some of the most popular and long-lasting mods include:
Ascension: Created by Bioware design director David Gaider in his spare time after the game's release, Ascension completely revamps Throne of Bhaal. The mod includes bugfixes, upgraded difficulty and AI for almost all of the bosses in the expansion, additional dialogue for Imoen and Sarevok (including a new conversation if you carried over his Sword of Chaos from II's opening dungeon), an upgradable Slayer experience tree, the ability to convince Balthazar or Bodhi to assist you during the final battle, and extended epilogues. Additional fan mods tied into Ascension include Turnabout, which allows the player the option of summoning additional NPC's (up to and including plot-mandated dead squadmates, Gorion or the Player Character's mother), and Redemption, which allows you to recruit (and redeem) Jon Irenicus as a party member.
Unfinished Business: Comprised of side missions, extra dialogue and gameplay encounters that were cut and not properly implemented into the game. The fanbase used script remnants, interviews with Gaider detailing the cut material and voice files to reconstruct the missing elements, which include a Minsc side mission, dialogue between Valygar and Suna Seni, restored character creation kits, missing items and NPC portraits, and more. A similar mod of restored content also exists for the first game.
Kelsey NPC: Far and away one of the best and most popular NPC additions, Kelsey is a sorcerer from a family background of successful merchants. He is fully voiced, with banter options with all the standard NPCs, a romance option for female PCs, a fair number of sidequests, and is fully playable through the end of Throne of Bhaal.
Baldur's Gate Trilogy: One of the few so-called megamods, BGT is designed to take all of BG1's content and convert it into the BG2 engine. This, along with some added dialogue options, allow both games, along with their expansions, to be played as a single massive game. This has also opened up many modders to add content to BG1, whereas it was originally neglected as being difficult to modify. But with BGT installed, both games run on the same engine and thus modifications based on the sequel's engine can be introduced into the original game. In the same way, NPCs can be introduced in the first game and be made to run seamlessly through the entire series.
Big World Project: An insanely ambitious mod still in production, the BWP is less a mod and more of a toolset with a number of utilities designed to do no less than let the player download and install any combination of hundreds of available mods, resolve all conflicts between them, patch some mods with others, and debug the entire shebang. The BWP downloads the mods for you, installs them for you, debugs and deconflicts them for you, and despite still being in development, can install several gigs worth of WeiDU-based mods with minimum difficulty. Though if you're installing any appreciable number, the entire process can take several hours to complete. It is, quite simply, the ultimate in ease-of-use, hands free mod installation for the Baldur's Gate series.
Infinity Animations: Less a single mod and more a Baldur's Gate modding sub-community in of itself. Infinity Animations is group of graphics modders dedicated to improving the appearance and quality of the many paper dolls and creature animations throughout the game. They are also responsible for importing creatures from the graphically superior Icewind Dale and Planescape: Torment games to take the place of their outdated Baldur's Gate counterparts. Thanks to their work, there are now paper dolls reflecting the many race, gender, and equipment combinations available, allowing for a truly customizable character appearance. IA also serves as a central clearinghouse for mod-created creatures, ensuring that there are no duplicate creature creations in the database and designating unique file assignments so there are no overlapping file names, allowing a player to download any number of IA-approved mods without having to worry about deconfliction issues.
Throne of Bhaal Extender (TobEx): A popular pre-loading program designed to be loaded with the game itself. It is difficult to explain precisely what it does, per se, but it is designed to soft code fixes directly into the game's executable in order fix problems and make changes to the engine itself as the game is played. At present, it fixes the infamous "stutter bug" caused by an overabundance of global event counters collected in long-running games. It also modifies the character creation process and other screens to insert a scroll bar where none existed before, allowing for a theoretically infinite number of kits and spells to be selected where before the only ones available were those that could fit onto the static screen. Dozens of other "behind the scenes" mechanical fixes are also present for game stability. TobEx's work is so popular that its creator, Ascension64, is one of the lead fan contributors to the game's 2012 official relaunch and most of TobEx's features will be included in the revamped engine.
BG2 superfan Wesley Weimer famously created the WeiDU interface, which was a hybrid script / text file extender that could be used to add modular pieces of new content to the existing game. In addition, Weimer created several well-known mods for the game, including Solaufein (which adds the Drow from Ust Natha to the party), Item Upgrade (which allows for new items, weapons and armor to be crafted) and Tactics (which greatly increases the difficulty and variation of enemy encounters).
Both the original Neverwinter Nights and its sequel came with all the tools needed to build new adventures. This is because they were conceived as "D&D on your PC": A DM would make a campaign, use the modding tools to build a session, referee it as a hosted netgame on his computer, and the rest of his group of players would all log in and run it from their computers, sort of like a virtual tabletop. The original Neverwinter Nights is probably the king of this trope. It's been out for years, and the toolset is considered, at least was considered, to be one of the best, most complete ever created. Essentially, you can make (and some people have made) content at least as good as the original and expansion content, in terms of complexity, depth, and detail—case in point: A Dance with Rogues.
Culminating in Baldur's Gate Reloaded which recreates the entirety of Baldur's Gate with the Tales of Sword Coast North expansion as a Neverwinter Nights 2 module (though with the standard 3.5 DND game mechanics, classes, and races available in NWN 2). GOG.com, which sells Baldur's Gate, even featured the mod on their site.
Dragon Age similarly spawned an active modding community, which is somewhat fitting for the Spiritual Successor to Baldur's Gate. Dragon Age: Origins was released with an official toolset (perhaps due to the success of the BG and especially Neverwinter Nights modding community), and as such countless mods are available for Origins and its Expansion PackAwakening. These include: major bug fixes, gameplay tweaks, hi-res texture packs, complete overhauls of NPC models, new appearance options for the character creator a la The Sims, new arms and armor, additional spells and classes, new quests and playable content such as party members and expanded Romance Sidequests. Many appearance-based mods are meant to increase visual continuity between the games, such as the various mods that change qunari and elf models to resemble their redesigns in the sequel, and Grey Wardens of Ferelden, which adds the sequel's Grey Warden armors to the first game. Dragon Age II did not come with a toolset, so fewer mods are available for it, but there still quite a few around, mostly new armors and character creation options for Hawke. And similar to the World of Warcraft example below, fan mods such as The Winter Forge, and Universal Dye Kit, both wildly popular armor customization mods, are probably why the developers were quick to tout the ability to customize armor as one of the first gameplay features revealed for Dragon Age: Inquisition.
The Elder Scrolls series has had tens of thousands of mods created for its various installments, starting from Morrowind onward. It has amassed so many mods that dedicated sections for each game are available on the NexusMods network, comprising new quests, weapons, armor, companions, enemies, complete visual overhauls / texture replacers and much, much more. The development of the TESCS (The Elder Scrolls Construction Set) jumpstarted the franchise's mod revolution, and the series gained a reputation for being extremely mod-friendly as a result. The PC version of Skyrim was used as the pilot title for the Steam Workshop, an addition to Steam's social network that makes uploading and installing mods a one-click process. Bethesda themselves have also released a handful of official mods for the series, most recently with the Valve Software-partnered Fall Of The Space Core, Vol. 1. Some of the most notable mods include:
Arguably one of the most ambitious Morrowind mods of all time, the Tamriel Rebuilt mod aims to add the entirety of the titular province as a playable area. An ongoing project since 2003, the mod is still being actively updated, having seen its last major release in 2012. To date, its roughly tripled the size of the game world, and it still has 3 more (out of a total 6) landmasses to complete. About the only two mods that can beat it in the ambition department are the two other semi-related (partly overlapping developers, a willingness to talk and share resources) province mods: Skyrim: Home of the Nords (aims to add the province of Skyrim as a playable area) and Province: Cyrodiil (aims to add the entirety of Cyrodiil as a playable area). Having begun much later, those two have thus far only released smaller showcase areas.
Both Oblivion and Skyrim have several mods comprised of comprehensive patches to both the base game itself and every major expansion or piece of Downloadable Content, thereby eliminating any discovered bugs not fixed by the developers.
Morroblivion, an ambitious project that fully recreates Morrowind on Oblivion's game engine. The creators also intend to accomplish the same feat by recreating Morrowind on Skyrim's engine. In both cases, you have to own both Morrowind and the game the mod is running on (by design - they're not trying to be pirates).
Nehrim: At Fate's Edge is a total conversion for Oblivion that turns the entire game into a mix between the original story and Gothic, complete with an entirely new world map and storyline that runs upwards of forty hours. It won the ModDB Best Singleplayer Mod award in 2010.
Skyrim has dozens of additional followers crafted by modders, although two in particular stand out from the pack. Vijia in Skyrim (the sequel to an Oblivion mod) introduces a female fighter / prospective bard to the game, complete with thousands of lines of dialogue (and contributions by Sir Terry Pratchett himself), a full storyline, morality system, tons of unique gameplay options and much more. Hoth is a fully-voiced bounty hunter who sports unique and custom-made armor, weapons and skin textures, and has his own storyline and side missions.
One of the most famous cases is Bethesda and their SDK - both the aforementioned The Elder Scrolls Construction Set and the aptly-named GECK for Fallout 3. In each case, the SDKs released are the same tools used by Bethesda to develop the game. This is a big, big part of the reason why Bethesda games are known as modder-friendly.
Many Fallout mods are also dedicated to fixing some of the inaccuracies and retcons compared to the old games, such as Advanced Enclave Power armor from Fallout 2, and many weapons from the earlier games.
A particularly impressive mod that's been made for Fallout 3 and New Vegas is RTS, where you can take virtually any NPC or object and move them to a town that you build yourself. Essentially making it into an incredibly violent post-apocalyptic The Sims-style game.
Like its predecessor, Fallout: New Vegas has a thriving modding community. Some of the more popular mods are New Vegas Bounties, a two part quest mod series lauded for its excellent story and challenging combat, Project Nevada, a complete overhaul of New Vega's gameplay, and New Vegas Enhanced Content (NVEC), a sort of "supermod" that takes many, many of the community's most popular bugfixes and enhancement mods and brings them together under a single mod.
Killap's "Fallout 2 Restoration Project" adds tons of material, mostly stuff that was cut because it couldn't be completed on time (such as rescuing Sulik's sister, the EPA, a secret Hubologist compound with loot, and several ending messages including a positive end for the deathclaws), but also multiple bug fixes and various minor improvements.
The only problem with the Megamod and the Restoration Project is that Fallout 1 and 2 were not designed with mods in mind, so the mods are somewhat unstable... but at any rate still better than unpatched Fallout 2.
Freedom Force has much more fan-created content than official add-ons, most famous is The Strangers witch is about the size of the original game and features much more technically advanced missions and powers. One mission features a four way battle between The Strangers, a pod of atlantains, a berserk shape-shifting robot, antarctican demon hunters and the Legions of Hell.
Romancing SaGa 3. So many hacks were made to that game to include characters from various anime, video games and manga — even other SaGa characters, and even adding bosses from the previous Romancing SaGa games and Apollo from SaGa 2. On top of that, the hacks even change the music and add new spells and scenarios.
Hacks also include a new boss from the Romancing SaGa Ultimania: The Mother of Death, Saruin, Schirach, and Elore; Saiva the Goddess of Destruction.
The PC version of Final Fantasy VII has a very active modding scene; the port is notoriously bad, being hindered by a staggered development cycle and numerous incompatibility problems. As such, the modding community has come up with a variety of patches and tools solely for fixing the game; there's even a custom-made graphics driver. There's also a huge volume of superficial mods, offering replacement character models (for both field and battle areas), high-definition textures, total menu overhauls, a complete retranslation effort and a tool to replace the in-game MIDI files with custom music.
Final Fantasy X is perhaps the most user-friendly of the sequels; barring the superbosses in the Coliseum, the game shouldn't give you any trouble if you know what you're doing. The PAL version rebalanced the difficultly somewhat, but for some real scares, try firing up the "Punishment" mod. Even former pushovers (the ones who croaked if you so much as lobbed an Aeon at them) now require a truckload of buffs.
While most mods to the game are discouraged as cheating and violate the Terms of Service, Final Fantasy XI PC versions are a frequent subject of mods. The least invasive and generally considered most benign (and easiest) involves modifying the game's data files to change the music for various areas or the visual appearance of monsters, NPCs, or player characters. There is a fairly significant mod community dedicated to this. On the somewhat more invasive level, a third-party application exists to provide one of the most heavily-player-demanded features, the ability to run the game windowed instead of full-screen, and supports a plugin interface to load additional mods into the game. This program is both highly popular and highly controversial, with many players in stark disagreement as to whether the bundled plugins count as cheats (though there are other plugins and third-party apps that are more generally accepted as obvious cheats) or simply features that should have been included in the first place. Square-Enix has a policy of banning players who openly admit to using it, and occasionally breaks it with updates, but has not made a concerted effort to address it, and has recently caved on the player demand for Windowed mode, and released it.... without all the mods they say violates the ToS, leading to the usual reaction.
The original Final Fantasy Tactics was practically tailored for modders, and not just for customizing sprites. For starters, the AI was terrible, the difficulty was broken, and - most bizarrely - enemy units never upgraded beyond knights and archers. The "1.3" mod ramped up the difficulty (well, it is hosted by insanedifficulty.com), strengthened the enemy units, and tweaked mages to be a little less, well.. useless. This mod definitely qualifies as Nintendo Hard; the days of finishing off Altima with two Climhazzards are over.
The Knights of the Old Republic games don't have any official tools, despite using basically the same engine as Neverwinter Nights; this, however, didn't stop fans from developing third-party modding tools and making a cornucopia of mods of varying quality, some even altering/"enhancing" the game, as well as the addition/alteration of some rather epic side/main quests. That's not even counting the additional items, restored content, models, skins, and bug fixes, etc. for a game that didn't even have an official toolset...
A couple of famous mods are a dialogue pack that restores much of Juhani's dialogue and adds about 20 pages worth of purely fan-made dialogue. The top of the line, however, is Silveredge9's Brotherhood of Shadow which includes three new recruitable characters, fully-voiced dialogue, at least a dozen all-new locations, flashbacks to your hunt for the Star Forge as Darth Revan, the fateful battle where you got captured, and single-combat against Mandalore at Malachor V! The only drawback is that, due to its magnitude, it will not play nicely with other mods.
Similar to the Baldur's Gate example is "The Sith Lords Restored Content Mod", a fan made mod that consists of a wide array of material that was cut from Knights of the Old Republic II when it was rushed for a Christmas release, along with over a thousand bugfixes. The mod was years in the making, and was finally completed in July 2012.
Visual Mods are very popular among World of Warcraft players. So popular that Blizzard has worked some of the mods into the main game, such as Raid Assist (which helps with said Raids by designating targets and tanks) or an option to speed up quest descriptions. Other mods provide additional information on items or change the UI (User Interface) to the players likings.
World of Warcraft is entirely supportive of addons that function entirely within provided scripting commands. On the other hand, data mods — even seemingly-innocuous client-only mods like model changes or (sigh) nude patches are forbidden.
Which are banned for good reason. If one were to alter models they could set it so say... the flag in PVP Battleground was a mile makeing it easy to find the enemy flag carrier at any time.
And many of those popular mods that were made into the main game (Enemy stats, Talent previews, quest tracking, raid warnings) have been in the game for so long most players probably won't remember what playing the game was like when those were only available with mods. (When, after patch, every single mod had to be updated...and these weren't just content patches that'd render the mods useless...There are numerous bug-fixing patches)
Not that it's stopping perverted players of course. Within a week of a data trick to allow a nude mod to work being locked out with a patch, another one will surface and the naked Goldshire mailbox dancing begins anew.
The classic BBS door game Legend Of The Red Dragon was infamous for its ability to use plug-in style mods, which were referred to as IGMs (In Game Modules). A great many of these simply heaped treasure and items on players for free.
The first three generations of Pokémon can be modded with some ease thanks to the proliferation of ROMs and hacking tools.
There's also the infamous hack Pokémon Quartz, full of bizarre new Pokemon and some really weird dialogue. Despite its flaws, it has fully edited areas and new scenarios, which was actually impressive for the time since Pokémon hacks then were mostly just graphic or text edits.
Nowadays, most Pokémon hacks are a lot more than graphical or text edits, containg completely new maps, scripts and, in some rare cases, new mechanics and completely rewriting the game. Pokémon Brown and the other hacks of Coolboyman are famous for doing the latter. Rarely is a Pokémon hack completed, however, with most "hackers" getting bored very quickly.
Pokémon also provides an in-universe example - Porygon-Z is the result of applying the Dubious Disc, an unauthorised third-party mod, onto Porygon2. Given that Porygon-Z is said to be glitchy and unstable, perhaps it's a Take That aimed at game mods in general?
Well the evolution is more minor then most. It essentially trades more defense for more attack, so its more like your trading what the creators made for something you find more useful/enjoyable. Likewise, the pokedex entry states that they're prone to random behavior but are still valid Pokemon to train, which could mean that mods are prone to crashing or screwing up but still something you can use effectively. Less of a Take That and more of a nod to Pokemon's very big modding community.
There are two hacks that allow you to catch every single Pokémon in it, by giving any Pokémon species some place to be found in. Said hacks are namely Pokémon Emerald 386 for Pokemon Emerald and Pokémon Platinum: Enhanced Edition for Pokémon Platinum. While 386 adds nothing (aside from the aforementioned possibility to catch every pogey in-game) to its source material, Enhanced Edition is the original game turned Up to Elevennote however, the hack doesn't feature the pogeys' locations in their Pokédex entries, such as, say, Squirtle's whereabouts being listed as "Unknown area", while 386 included every Pokémon's location instead: more challenging, with several trainers other than you taking a level in badass as well, thus requiring players to be at least strategically skilled and with a good enough knowledge of the good ol' Elemental Rock-Paper-Scissors. This includes being able to catch wild Mewtwo(s) even before getting to Victory Road, Route 201 entirely devoted to catchHM slaves, and the rematch against Cynthia revealing a party entirely made of Olympus Mons.
A group from /x/ has set out to mod Pokémon Fire Red into nightmare fuel for everyone's enjoyment.
Freelancer has some rather good mod communities, notably Crossfire, which actually makes the economy dynamic like it was supposed to be when the game was announced, Discovery Freelancer, which adds more content and optimizes certain elements for multiplayer roleplay, and Freeworlds, a total conversion of Freelancer into the Star Wars universe.
Swords of Xeen is an interesting case. A mod of Darkside of Xeen, it received official support from New World Computing and was included in some compilations of the saga, even if it's not considered canon.
An alpha version was leaked, and is available about the internet, but the version that was 98% done is not in public hands. However, a 99% complete version is.
There is also a video playthrough of the 98% complete beta available to view on youtube (http://www.youtube.com/user/CEMemorial), enabling you to see the interesting story that the creators constructed.
ROM hacks of Final Fantasy Tactics are also fairly popular. Most of these balance out overpowered equipment (such as making Excalibur's Haste effect temporary instead of permanent), give previously useless equipment like most daggers a reason to be used via stat boosts or special effects, balances out overpowered classes (I'm looking at you, Orlandu), and gives story battles more varied units to fight against. And then there are some hacks that seem solely dedicated to attempting to reach Platform Hell levels of difficulty, with varying success. You can find many of them here.
Torchlight has support for mods out of the box, as well as an editor, and achievements for playing the game with one, five, or ten mods installed.
Mount & Blade and its expansion Warband have a very large, very active modding community. Hundreds of minor mods and dozens of total conversions have been made, and more are released all the time. There's even a Star Wars mod in the works.
Titan Quest has its own modding community, although one mod gets recommended even to first-time players in lieu of the baseline. Consistently. That would be Underlord, which started out by, like Baldur's Gate's Unfinished Business mod, reinstating content that couldn't be implemented in time for Immortal Throne's release, and wound up tweaking and expanding all the masteries (and completely remaking at least half the Thief mastery, now Occult) so all the mastery dyads possible were actually viable. There's even been a mod, Lilith, that's essentially a completely new game that just happens to use TQ's engines.
Be warned, however, that Underlord also severely amps up the difficulty, to the point that the starting zone monsters can kill you. Easily.
Marvel Ultimate Alliance and X-men Legends 2 have a community found here dedicated to making all sorts of mods from fixing balances, adding new abilities and even custom characters that were not in the games initially for various reasons.
Unlimited Adventures is a somewhat meta example; the program is a Game Maker, so making mods is the whole point, but some people have figured out ways to create "hacks" which give the designer possibilities far beyond these offered in the program (editing items, for example.)
Despite the Mass Effect franchise never getting a set of mod tools released, fans took it upon themselves to create their own homebuilt tool, ME3Explorer, and start building their own mods.
Most famously, the Mass Effect 3 Happy Ending Mod (Extended Cut required) rewrites the ending of the game to have Shepard survive the the firing of the Crucible, via him/her never meeting the Catalyst and getting rescued by a shuttle from the Normandy. The most recent version of the mod has the ability for Shepard and their love interest to hug at the Normandy's memorial wall, along with new slides of other characters not mentioned in the epilogue.
There are also hi-res texture packs available for many of the environments and characters in the game, along with alternate outfit packs for squadmates and Shepard.
The "Mother of all Mods" unlocks a number of extra settings and customization options in the base game, including a keyboard manager, extra casual outfits and armors for Shepard, unused heavy weapons and in-game cheats.
Dark Souls had its fair share of mods after being ported to PC. It's mostly new textures for weapons, armors and characters (including the obligatory nude mod), some HUD changes and fixes for better connectivity.
The second most played mod after Counter-Strike, and also the second most played non-new release or MMO game online (after Counter-Strike) is Defense of the Ancients: All-Stars, or DotA. It has become far more popular than normal-style Warcraft 3 multiplayer matches.
Once upon a time Warcraft II had a mod called Kali, with the esoteric function of emulating a local area network between all players who were using Kali across the Internet. Blizzard took notes and built Battle.net.
The Europa Barbarorum mod for Rome: Total War. Began with the intent to revise the game scenario to be as historically correct as possible. The current 1.2 version is a hugely ambitious mod incorporating retextured units, an extremely detailed trait system intended to simulate various aspects of cultural and political life of the time, building descriptions giving considerable amount of historical background information and, perhaps most notably, having original unit voices and commands replaced with new ones in Latin, Attic Greek, Gallic, Q-Celtic, Punic, Pahlav and Proto-Germanic. They have even offered their expertise to Paradox Interactive for the title Europa Universalis: Rome.
There is also another mod, Rome: Total Realism, which aims to do much the same. Explaining the differences between the two would take up this whole page, and in the end which one you use is a matter of personal preference. Both are excellent, however.
There is also a very popular mod based on Lord of the Rings called the Fourth Age Total War. This mod takes Tolkiens abandoned manuscript called 'The New Shadow' and runs with the concept by not only interpreting Middle-earth as it may be in a Total War scenario, with Gondor dealing with it's people turming evil. But it is also filled it with tons of references to Tolkien's work and keeping it true to the spirit of the stories. Even six years since Rome: Total War was released, this mod is still being worked un and updated due to the developers insistance on detail, making sure it is released as bug free and finished as possible and also due to the flexibility in design that Rome was. Currently the team are on version 2.6, which has the factions of Rohan, Dunland, the Gondorian regions of the Reunited Kingdom and other factions which have all be fleshed out and fit into the world. Due for release this year is version 3.0, which will add Elves, Dwarves and Dale as playable factions, as well as opening up the western and norther regions of Middle-earth, including the lands of Arnor, the northern part of the Reunited Kingdom.
The aforementioned Third Age has become what many consider to be one of the biggest and most comprehensive mods ever. It transports the base game from medieval Europe to Middle Earth. This is complete with hundreds of new units, generals, and game mechanics meant to simulate Middle Earth.
A hugely ambitious mod for Napoleon: Total War -a game that is far more difficult to mod than its predecessors- sets the game in the First World War. It includes period accurate artillery, gas weapons, flamethrowers, tanks, and mass swarms of infantry. It can be found here.
For every Total War game released, savvy players know it's only a matter of time before a bunch of modders get together to put together and release a mod that aims to correct any historical inaccuracies and gameplay issues.
This includes the most recent Total War Rome II, which already has mod tools, Steam workshop support, and many new mods.
When Warhammer 40,000: Dawn of War first came out, fans knew that they weren't going to get all of the 10+ different armies (not counting sub-armies) that are available for the tabletop wargame. So naturally, mods to put the other armies into the game were begun before the game was even finished, as soon as the beta was released. Of course, due to the Expansion Packs that came out late, nine of those armies presently are are or soon will be in the game, including many ones that had mods in progress. This doesn't stop the modders, who are shifting to gameplay tweaks, adding units not in the game, and making present units closer to their tabletop abilities.
The only army that isn't in Dawn of War is the Tyranids. Although, they are in the sequel (and I hope that the mods to add Dark Eldar/Sisters of Battle aren't as awful as Soulstorm).
Various Command & Conquer games have Total Conversion mods that usually use a later engine for an older game: Tiberian Dawn on the Tiberian Sun engine, all previous games on the Generals engine, Tiberian Sun (Reborn) and Red Alert (A Path Beyond) on the Renegade FPS engine, and I didn't mention Tiberium Wars yet.
In fact, since Red Alert 2 uses a newer build of the Tiberian Sun engine, it is perfectly possible to copy-past the latter's content into the former with some INI editing.
The Tiberian Sun/Red Alert 2 engine is practically a modder's wet dream, with all unit, etc. parameters being written into INI files in plain English and requiring nothing more than Notepad. While many things are hardcoded into the executables and are thus untouchable, modders found a way around that too with RockPatch (buggy EXE hack), NPatch (even more buggy EXE hack) and more recently Ares (DLL injection, still in alpha version).
Likewise other games from the developer (Paradox Interactive) have a lot of mods as well: Europa Universalis 2 has the massive A.G.C.E.E.P., and Europa Universalis 3 has the slightly less Massive Magna Mundi (and a host of others). Paradox make life easy for the mod community by storing most of the game data in text files, so they can be edited with nothing more complicated than Notepad, and have dedicated sections on their official forum.
It's not surprising that Total Annihilation had a large number of mods, including total conversions, given the game was popular; it is rather more surprising that its much less successful sequel, Total Annihilation: Kingdoms, had just as many. The versatility of the engine often resulted in additional races or total conversions that many believe are better than the original game.
Let's not forget Spring, a TA-inspired engine positively saturated with great games (most of them free) that are all referred to as "mods".
A strangely popular pursuit is taking one RTS game and making a total conversion mod to essentially turn it into another RTS game 'as seen through the first game's engine'. For example, there are mods that turn Command & Conquer: Red Alert 2 into Red Alert 1, Total Annihilation into Red Alert 1, Total Annihilation: Kingdoms into Cossacks: European Wars, and so on and so forth.
Also, attempts are being made to make StarCraft run on Linux by using the Strategus engine. Strategus doesn't support free-roaming units, idle animation, critters, or many of the mechanisms used in Easter Eggs at all, and the creep, Pylon power, and building liftoffs would require at least some minor modifications to the engine itself... good luck with that.
Jagged Alliance 2 has v1.13, whose name is derived from v1.12 being the last official patch. The mod, continually updated to this day, which completely revamps the game and even adds an online multiplayer mode!
Some of the gameplay changes such as the New Inventory system (revising characters' gear so that they would have to wear Load-Bearing Equipment "containers" such as vests, backpacks, and rucksacks), the revised equipment system and the 100-point Action Point system were great.
To get an idea at how huge this mod is, it adds in around 725 new firearms into a game that originally had around 20.
The Unfinished Business expansion was built specifically to support new campaigns. It even included tools for editings things that were previously hard-coded.
Two major mods were created by hacking the original game's source code. They are Urban Chaos and Wildfire, both developed simultaneously by two different teams for several years after the release of the original game. Eventually, the company that owned the rights to JA2 at the time offered both teams the chance to release their mods as commercial expansions (similar to the Half-Life/Counter-Strike deal). The Russian team that made Wildfire eventually took the offer, their mod was released commercially, and the package included the source code for the game executable (published under a semi-open license, paving the way to 1.13).
By 2010, Jagged Alliance 2, Unfinished Business and 1.13 have several dozen campaigns available between them, not to mention many MANY item packages, map packages, and other enhancements. JA2 is easily one of the most heavily modded games released before the year 2000, overtaking even such games as Fallout in sheer volume.
For a game released in 2003, Homeworld 2 has a lot of mods. These come in two flavors, discounting total conversions:
The first are mods that aim to enhance how the two races play out. The most popular are arguably the Complex Mod, and the Point Defense System (PDS) mod. Both added tons of extra features and customizations, essentially making the game more complex (especially in, well, Complex, where rank, soldier population, and morale system is introduced). PDS started out as...a small mod for point defense systems, which became the popular mod they are now. The Tactical Fleet Simulator mod aims somewhat differently unlike the former two. While Complex and PDS emphasizes grand strategy with a lot of decked out units and careful planning, TFS gives decked out units purposely built for intense, fast-paced combats which ups the "tactics" aspect more than "strategy" aspect, which the game is already good for anyway.
The second flavor are mods that try to insert additional races, while mostly retaining original units. These can either mean original units, or factions from the original Homeworld. So far, the most successful one is the Chinese-made FX mod, which adds almost all of HW 1's factions, some of them having completely new starships. Their crowning awesome is that they added the Progenitor race from HW 2, which, in-game, only consists of 4-5 separate units at best. They expanded the race using both unused concept sprites, mishmashed existing units (which they did VERY well), or design from scratch.
Note that there are literally hundreds, if not THOUSANDS of mods for Civilization IV. Firaxis, the developer company, actively encourages modding. In fact, several mods were developed by dev team members themselves and came bundled with the original game, including Next War (a heckuvalot like 1984) and Final Frontier (a space-based mod with an original storyline, which has itself been modded for various science-fiction things, such as Babylon 5). Additionally, "Rhye's and Fall of Civilization," the Civilization IV form of the great Civilization III mod "Rhye's of Civilization," was also bundled (as I recall), despite being a fandom mod.
Not only are there mods of Civilization IV, there are modmods of these mods, and on top of that, modmodmods.
Civilization V is continuing the trend, though the ability mod is somewhat limited until the promised .dll source files are released. In the meantime, there's an in-game mod browser, including the Vanilla Enhanced Mod (game balance), the Unofficial Patch (bug fixes), and NiGHTS (reworks the whole game; no relation to the Sega game of the same name). There are even mods that cross the boundaries of designer, genre, and setting, such as bringing in Asari, Geth, Quarian and Turian civilizations, although weirdly enough most of the units still look human.
Master of Orion 3 coded an incredible amount of its game data in easy-to-modify spreadsheets. Given the number of Game Breaking Bugs and incredibly odd mechanics in general, many players found a modded game the only playable version.
Magna Mundi, a popular mod for Europa Universalis III, recently got an upgrade to "full game", developed by the mod team but for commercial release instead.
As did Arsenal of Democracy and Darkest Hour for Hearts of Iron 2. In fact Paradox encourages moders to form teams that will create something for them to publish.
Magna Mundi has since been cancelled due to various reasons.
Red Alert 3: Paradox is a highly ambitious Red Alert 3 mod notable for it`s accompanying wiki project, which is slowly building a detailed 1960s Cold War world out of the highly limited basis of the original game.
The Space Empires series from III onwards. III shipped with a modding tool, while IV and V run on text files.
The 4X game in space game Galactic Civilizations 2: The Dread Lords, especially after the expansion packs are added, can be modded quite significantly. Mods can include new technologies, whole new tech trees, weapons, planetary buildings, ships, races, and more. There are many total conversion mods that turn it into Star Trek, Star Wars, Babylon 5, and more. The game's data is stored in organized text files and can be edited using Notepad, however many editing tools, including some official ones, are available.
LEGO Rock Raiders was built primarily using .wad files, and so extraction tools have been made to reveal simple english code, easily changable models and animations, levels that editors were made for, easy images, etc. Now more building, monsters, levels, raiders, new models and animations, even overhaul mods are being made.
Myth II came with a couple of very good editors; it was actively modded for many years, and to some extent still is. In this player's experience, it has more stand-alone mods than any game made before 2000 except Doom, Quake, and Half-Life... and the fact that it's the only non-FPS on that list says something. Moreover, many of those mods (The Seventh God, Jinn, Chimera, Bushido) are large campaigns on par with the originals, and have been included as bonus content in commercial releases such as The Total Codex and Myth II: Worlds.
Starcraft-Broodwar has several mods as well, with the Gundam-inspired Gundam Century (ultimately having gotten killed while still in beta from a combination of disillusionment on the part of the creator and a cease-and-desist letter on the part of Bandai) being only one of them. Huncraft is an Hungarian-exclusive game mod, that works in the spirit of an actual expansion pack, only with fully translated unit speeches, menus and tooltips.
Advanced Strategic Command have sets ranging from Battle Isle recreations to its own "more realistic" group, and beyond, being optimized for easy creation of rulesets.
Even if you can set one sprite per weather, a custom sound for each weapon, etc, only one sprite per unit/building/object/terrain is really necessary and abstract elements of Tech Tree don't need even that. Data packs are additive and can be cherry-picked on each map, and raw game data other than sprites and maps is all written in plain text and with object inheritance, so properties universal for e.g. all infantry, all drydocks or all anti-air cannons are written only once, in parent class. And what you can't do in data, you can do on a map by using Lua.
FTL: Faster Than Light has an impressive modding community, with an entire sub-forum on the official forum just for mods. The mods range from purely aesthetic mods (such as "Better Planets and Backgrounds"), to new ships you can play the game with (anything between "old-school fighter-planes from World War II" and "the TARDIS" have been created), all the way up to total-game-overhauls, such as "Captain's Edition" and "Descent Into Darkness".
Operation Flashpoint, a 'soldier sim' FPS by Bohemia Interactive was released in 2001, and still has addons and total conversion mods being made for it today. Its two sequels, ARMA: Armed Assault and ARMA II, maintain the modding community of its predecessor.
The sheer volume of content people created for the former bears special mention. The developers kick-started it by including the mission editor, which allows players to create anything from a simple run around and kill a few guys scenario to full-blown combined arms battles far exceeding the complexity of the game's default missions. People soon went from creating their own missions to creating new weapons, soldiers, vehicles, and eventually whole new islands and a new Capture the Island mode, which is a pseudo-RTS game mode in which players battle for an entire island, using resources to build bases and buy equipment). In ARMA II the most "out there" examples would probably be naval combat and in particular submarine warfare of all things, both of which were probably never intended to be implemented by the devs... yet the modding community found a way. There's also been "cops and robbers" and Grand Theft Auto-style Wide Open Sandbox "missions". The DLC for ARMA II: Operation Arrowhead was actually designed to facilitate this: British Armed Forces and Private Military Company consist of their respective campaigns and higher-quality textures/sounds for their respective units and weapons — which were already in OA, so that add-on creators could make use of them even without having the DLC.
The Sims and its sequels allow easy creation and importation of new clothes, faces, and furniture. Website after website can be found with downloadable items that players have designed themselves. Briefly, The Sims 3 was changed to make it mod-unfriendly, but a later update changed that, if not back to the original mod-friendly design.
FreeSpace 2, thanks to the release of its source code by Volition (under the Freespace 2 Source Code Project), is a frequent subject of Game Mods in the space sim community. The original relase's multiplayer is still functional with dedicated server hosted by the community.
SimCity 4 has quite a bit of mods available at Simtropolis. These mods can go from simple stuff like making the streets' dead-ends more rounded or making the water darker and more realistic, to downright cheating like ordinances that completely erase all pollution and crime from your city, to simple building addons, to entire expansion sets like the Network Addon Mod that adds avenue interchanges, diagonal-straight highway interchanges, roundabouts, walkways, train-street stacks, light rail and more, to completely new games like the SimMars mod that sets the game in Mars.
The mid-90s game Transport Tycoon Deluxe no longer even works on most computers without the help of DOSBox, due to its DOS-based format. However, OpenTTD is an open source emulation that still attracts lots of Game Mods, from replacement graphics to entire new industry chains, alterations to the interface and even whole new modes of transport (tramways recently added to complement the buses/trucks, trains/monorails/maglevs, planes and boats already in the game). One of the mods, to begin with, replaces the locomotives' in-game names with the real ones (e.g. "TIM" and "AsiaStar" are replaced with "TGV" and "Eurostar").
The Zoo Tycoon series has a large amount of fan-made mods made for it, usually to add creatures not included in the game (such as dinosaurs and the denizens of The Future Is Wild series). The first game even had an entire mod creation program created for it — since the animals were sprite-based, it was fairly easy to recolor them if you knew how.
Tons of custom content is around for Creatures and its sequels: objects, creature breeds, and occasionally new worlds/add-on rooms. Official tools to make CC have been available nearly from the start.
The simulation game Petz for PC has a dedicated modding community full of people, called hexers, who use hex-editors to create their own breeds of Dogz and Catz - and even other creatures, from Horsez to Dragonz. More recently, a hex-editor called LNZ Pro was created especially for Petz editing, and a tool called "Petz Workshop" made it possible for users with no hex-editing ability to visually design new Petz breeds in a user-friendly interface.
This is also true of the sister game, Babyz, which runs on the same engine and is editable with the same tools. Although, most mods in Babyz tend to focus around making toyz, clothes, and hairstyles instead of whole new body shapes.
Falling Sand Game 4.4, the original powder game, allows for its physics files(which are just text files) to be modified easily. A community has sprung up around modding the game.
Wing Commander Secret Ops is another favorite of the space sim fan community, for its relative ease of modifications. Its immediate predecessor, Prophecy, has even had multiplayer activated by dedicated fans, the core code for which was Dummied Out due to production time constraints (but not before an ad was published hyping its multiplayer capability).
Vega Strike got many mods in status from abandoned to frozen waiting for engine improvement to regularly updated. Includes Wing Commander/Privateer universe and whatnot. Aside of stats and models, has Python scripting for things like non-cockpit GUI,generation of dynamic universe and missions logic, XML-customizable AI and XML+Python stock missions.
Orbiter is essentially just a framework on which mods can be installed. There are literally thousands of them available, from ultra-high-resolution planet textures (running into the hundreds of megabytes per planet) to accurate-to-the-second-and-centimeter recreations of every single manned spaceflight ever launched from Vostok to the Space Shuttle and everything in between, flyable versions of pretty much every experimental spacecraft, test plane, and lifting body to ever end up on a drawing board, fictional ships from every sci-fi universe imaginable and plenty of original ones as well, building block kits for constructing your own surface base or space station....the list goes on. The mods considered most essential are Orbiter Sound, which does Exactly What It Says on the Tin, and UCGO/UMMU, which add a cargo system and a life support/crew system to the game respectively.
For the player extremely concerned with realism, there's a fanmade scenario floating around that includes the full constellation of active satellites in Earth orbit in updated-in-real-time accurate position. They would have included inactive satellites and space junk too, but most computers would have been unable to handle it.
With the right mods and enough time on your hands, it's possible for you to play out the entire history of spaceflight in sequence starting with Chuck Yeager's first supersonic flight and continuing through the decades up to the present day and beyond.
The mods have also extended to the creation of fanmade cooperative 'campaigns' in which players work together, each one flying a mission in sequence, to accomplish an objective; the frequent 'Orbiter Forum Space Station' campaigns are an archetypical example.
The open-source space simulator Celestia is also basically a frame to add mods, that range from more detailed textures or newly discovered celestial bodies to real or fictional spacecrafts and space stations, entire solar systems (also fictional or not), and even (pictures of) galaxies. Just search for the Celestia Motherlode as it's known.
IL-2 Sturmovik's avid global modding community makes lots of new maps, skins, missions and campaigns, addon planes and total conversions on a regular basis. Some more complex mods are a bit harder to implement into the game than others, but generally, the modding community has made mods for anything related to prop and jet plane combat flight simulation of the 20th century (particularly things from the WWI era up to the 1950s).
The WWII submarine simulation game Silent Hunter has a huge amount of mods, most of which are oriented to give even more realism to the game (and make it even harder). Plus they may add new ships or planes and -in the case of Silent Hunter III- you can even add sounds and short videos taken from the movie Das Boot.
The MechWarrior Living Legends total-conversion mod for Crysis Warhead - which adds in battlemechs, jet fighters, power armor, massive maps, and a huge array of vehicles and weapons. After a number of aborted sequels to MechWarrior 4, it became the fandom's MechWarrior 5
The X-Universe games contain a built-in script editor, and mods have been released that do everything from tweaking the AI to reduce Artificial Stupidity to (in one case) replacing the entire scenario with Babylon 5. Egosoft, the developer, is surprisingly open to mods, and for every game since X2, has released a Bonus Pack of signed scripts written and maintained by X forum member Lucike. This is important for two reasons. First, is that normally, adding scripts marks the game as modified, removing forum users from contention in rankings, which use in-game statistics uploaded to the forum. And second, having a modified game also disables Steam Achievements. Having the bonus pack scripts being signed, means they are 'vanilla' content and as such won't mark the game as modified. (Granted, those who are inclined to mod their copies don't generally care.)
In the X3 trilogy Egosoft often added content developed for fan mods into the main game, making them Ascended Fanon. The OTAS corporation in X3: Terran Conflict began life in a mod for X3: Reunion, and X3: Albion Prelude's 3.0 patch will be almost entirely composed of content developed by a modding group called the AP Community Project.
The Tachyon: The Fringe "Bloodstar Mod" would add a bunch of unattainable NPC ships to multiplayer, at the cost of replacing all the Bora & Galspan ships and breaking the single-player campaign.
Vector Thrust is designed ground up for mods with text-based game configuration files, extremely adaptive AI that can learn how to use custom-made weapons, and fully functional mission, map and campaign editors. Being a Spiritual Successor to Ace Combat, it's practically begging for its fans to churn out remakes of the classic PS2 series.
You can find a Let's Play for The Second Reality Project Reloaded, a remake of two separate ROM Hacks into one game, with the content from the secondary hack tucked away in a super-secret world. The author of the Let's Play in question also features his earlier Let's Plays of the two separate original hacks as a comparison.
There's also the infamous Kaizo Mario World, that Platform Hell codifier hack that basically started off all this 'ROM hacks for Mario have to be really really hard' trend (and the one which caused quite a bit of annoyance towards Youtube LPs by the aforementioned Mario World hack site).
ATA's Extra Mario Bros. is a technically impressive conversion of Super Mario Bros. into a nonlinear Metroidvania. The same hacker's Luigi no Nazo is a sillier hack of the same game, borrowing a lot of annoying features from Transformers Convoy No Nazo and featuring Multiple Endings. ATA has also produced hacks based on Mega Man courses, e.g. VS Airman which is clearly a reference to the Airman stage in Mega Man II.
The hacking community of Super Mario Bros. outside Japan has produced few notable works. Some notable hacks include Frantik's Super Mario Unlimited which improved graphics and added new items and collectives; and w7n's in-progress Rohrleitung Gate which enhances the music using FM channels and enabled free horizontal scrolling within 2 pages and parallex scrolling effects.
Nintendo themselves joined the scene in the Wii U game NES Remix, which alters Super Mario Bros. levels (and other NES games). Generally the mods affect physics, visuals, and player\enemy behavior rather than changing the level itself.
A special case is the Knuckles in Sonic 1 hack, which (as the name implies) makes Knuckles into the playable character in the original Sonic the Hedgehog, in a similar fashion to the Knuckles in Sonic 2 that is obtained by connecting Sonic the Hedgehog 2 to Sonic & Knuckles. What makes this interesting is that the original developers had tried and failed to put Knuckles in the first game during the making of S&K, citing technical problems. To make matters even more interesting, they've also created another version putting Tails into the game (with the spin dash, and flight).
Hacking has enabled anyone who's interested to play through all three original-series games as Amy Rose (using her Sonic Advance moveset).
In fact, there is a robust community of modders creating ROM hacks of the original series of Sonic games, ranging from the simple, such as recolored levels and simple character replacement (by modifying other characters' sprites), to the intricate, such as creating new level layouts and/or art, adding new characters with unique special abilities, adding new abilities to existing characters, adding new music (either by porting it from other games or creating it from scratch), and even porting or creating new boss characters; some more ambitious projects have arguably converted a given Sonic game into what qualifies as a new entry in the series.
Most notable is the heavily recoded Sonic Megamix, a romhack of the original Sonic, which features saves, 4 new characters besides just Sonic, a new soundtrack, new level art as well as secret 4th acts for each level, new moves and abilities, super forms, new bosses, and a load of other features such as a heavily expanded Sound Test, more options, and Time Attack modes (including a Boss Rush Mode). It's so good that people have likened it to a 5th 2D Sonic game.
The aforementioned Megamix has now become so expansive that it has been ported to Sega CD format, and will apparently (when the game is properly burned to a CD-R, of course) even play on an actual working Sega CD. Madness.
Sonic 3 Complete, the optimised version of Sonic 3 & Knuckles. This hack posits a version of Sonic 3 that was originally promised before Sega were forced to split the game in two. Including such features as:
Single player controlling of Tails' flight, including carry of Sonic, by using the Up direction.
Specialised activation of Super/Hyper mode, i.e. the player can't inadvertently activate it just by double tapping jump (as is necessary for all three characters), but instead by hitting C twice.
Consistent behaviour of monitors when attacked from below (original versions behaved differently).
Consistent music throughout games
This even includes swapping out the music for the Sonic and Knuckles Collection versions and the Main Theme/1-Up/Game Over/Continue themes to Masato Nakamura's original composition for the first two games.
Option to use Sonic 3's Big Arms final boss phase, cut from Sonic and/or Tails' Launch Base Zone act in original combined edition.
Improved Super Sonic sprite animation
Likewise, players are given the choice of using Sonic's old sprite from the first game or Sonic 2 (as was originally intended in the development of Sonic 3).
Knuckles' intro cutscene with Egg Robo logically occurs on Angel Island Zone rather than Mushroom Hill.
Optional opening of different paths for Knuckles for "research purposes".
The ability to change gameplay styles to match that of the original game, Sonic the Hedgehog 2, or Sonic CD (both the orignal and iOS versions).
Also allows the player a large degree of mix and match customisation for his/her ideal Sonic 3 experience.
That same hacker has now done several secondary characters in Sonic 1, including Amy, Vector and Charmy Bee. Charmy's game is a complete Game Breaker since (just like in Knuckles Chaotix) he can fly forever with no cool down period. Other hackers have managed to implement characters such as Shadow, Metal Sonic and Motobug!
Sonic the Hedgehog 2 has an interesting hack involving onion rings. While Sonic still needs these rings for protection, collecting too many makes Sonic gain weight and the fatter he gets, the slower he moves and the lower he can jump. This means players can't just blaze through the levels unless they want to collect all the rings along the way and make Sonic an immobile blob of fat. Speaking of immobile Sonic, he will die in a few seconds if he becomes too fat to move, possibly from a heart attack. Ring boxes also have to be avoided like the plague and Tails will make things worse for you if he keeps breaking them. Luckily, the rom hack has special items that resets Sonic's weight to normal and to prevent the game from being Unwinnable in the water sections, fat Sonic can jump high under water.
Mega Man Zero 3 has been the target of various well done sprite hacks that replaces Zero with a completely different character such as Vent, or Copy X. First started off as a hack that allowed the ability to play as Omega Zero which was basically a palette swap of Zero and giving the player access to his exclusive attacks.
Rockman No Constancy replaces the environments, attacks, and even the music: rather than Mega Man 2's futuristic looking stages, the ones in Constancy look more mystic or dreamlike (think Sonic CD). Think the game is easier? Think again. The bosses have faster attacks, the stages have brand new gimmicks, the stages themselves are new, and the scenery is just beautiful, as is the music (Yes, it's taken from various sources, but the soundtrack is beautiful anyway). For a peek, look right here.
If you wanted a hair-ripping hack, try your hands at Super Metroid: Impossible. This is made by the person who developed the under-two hour tool-assisted speedrun for Redesign, a mod that typically takes as long as Metroid Prime.
When it comes to reasonable hacks, we have Super Metroid Dependence which, for the most part, is pretty fair in its gameplay and really only has two points that come to mind (at least at the moment) where the player can become completely lost, one involving bombing a particular wall in Brinstar and the other that involves the creator being a dick by making a certain fake wall in Tourian not be revealed by the X-Ray Scope.
Can't find your way through the orignal Metroid? Hates having to write long passwords? Hates having to choose between Ice and Wave Beams? What about grinding for health after continuing the game? Well, my friend, someone was thoughtful enough and made us a hack that improves the original Metroid! That's right! It gives you not only 3 save slots, but also a map screen while paused and allows you to use Ice and Wave Beams together! And it will also save your health!
Eversion, having its source code in a text file for all to see, is very easy to modify. Common mods are designed to access its ten or so Minus Worlds or allow for flight or invinciblity.
The PC version of Prince of Persia had an even harder level hack called 4D Prince of Persia back in 1994. In more recent years, however, hackers became capable of deeper modifications, and now you can get many new level sets, tiles and character sprites - you may play with levels and graphics from the excellent SNES version. Someone even made a launcher to use all of the stuff combined.
Somari was a pirated Famicom port of Sonic the Hedgehog with Mario replacing Sonic. In turn, the company made a Doraemon total conversion of it.
Star Wars (Data East) got a software update created by fans and hobbyists that made major revisions to the rules, fixed bugs, and improved game balance.
Paragon was originally designed for four flippers, but tables for Europe were modified to only use three flippers, to better appeal to the preferences of Italian players.
Hobbyists and companies have taken Stern Pinball's NFL — which can already be customized with team-specific backbox translites — and further tailored them for specific teams and players, with new cabinet and translite art, additional lighting mods, and more playfield toys.
A large portion of the Super Smash Bros. community modifies pretty much everything within Brawl's engine, including custom stages, character skins, movesets, physics, menu appearance, and music. The mods listed below are the major full-on mod projects made from Brawl.
Brawl+ was the first attempt to make Brawl more competitive-oriented. While it reached version 7, the team eventually lost interest and moved on to create Project M, which by contrast is specifically meant to replicate Melee's gameplay.
"Balanced Brawl" tries to mod Brawl by keeping most of the engine intact (i.e. defensive play favored, very little hitstun) while trying to make the game balanced.
Brawl- enhances the game by making everyone God Tier, yet balanced so that it's practically acceptable. It has hitstun comparable to Smash 64, and it's about as fast as Brawl+. It's Brawlon steroids.
Then there's Project M, which basically turns Brawl into the next Melee, having the return of Melee's advanced techniques and physics engine. It's intended to even surpass Melee by making every character good, making almost every stage tourney-legal, and of course utilizing the benefits of being made from Brawl, like better graphics, more characters, and modding capability (see above).
Even without hacking, the arcade version allows players to play custom songs and custom stepcharts for pre-existing songs.
Most Dance Dance Revolution arcade cabinets can also accept custom stepcharts from either USB or Playstation memory cards. A hacked cabinet can also have custom stepcharts built-in, as well as allow modifiers like Fuwafuwa (usually found only in Oni courses) selectable in regular play, and change the color scheme/theme.
Tomb Raider has a level editor, which was released back in 2000, but to this day still has many users developing content for it. It is easy to use due to its Invisible Grid system, but this is somewhat restrictive and gives a very "blocky" appearance. The level editor is for an old PS1 game engine, but modifications and patching have allowed for almost-PS2 quality graphics. Notable levelsets include:
Tomb Raider 1: Revised and Unfinished Business Remake, almost perfect recreations of the original game and Gaiden Game keeping as faithful as was possible with the engine at the time (some things had to be modified or removed). They were created because the games were totally unplayable on most modern computers without the use of extra programs (which cost money)
Himalayan Mysteries, an Original Flavor epic that took five years to make, elaborating on the story behind the original continuity's claim that Lara was involved in a plane crash at age 17, rather than age 9 as stated in the Continuity Reboot.
TR Search HQ: Emergency, a bizarre, Painting the Medium-esque game in which Lara must stop the level editor itself from being hacked. Contains "British Mountain Ranges" just a few minutes away from Lara's mansion in Surrey.
Tomb Raider 4 Gold: Fading Light, notably based on the developer's concept for a Tomb Raider: 4 Gold that was cancelled.
Tomb Raider Anniversary: Retold, a fan-made remake of the original Tomb Raider which comes from fan disappointment with Crystal Dynamics' remake (Tomb Raider: Anniversary). The goal is to produce a more faithful and respectful remake of the original game, with elements of the cancelled remake that was being made by Core Design prior to the Continuity Reboot. After almost two years of development, the first quarter of the game has been released to mixed reviews due to flaws in the control scheme and dull (by TRLE standards) lighting. The project leader was a bit delusional about the quality of his work, and developed an ego problem. That's not a mean thing to say because he's the one typing this. It would appear that TRLE users hate it, while standard-fare TR fans adore it.
Ultimate Mortal Kombat Trilogy, a popular hack of the Sega Genesis/Megadrive version of Ultimate Mortal Kombat 3. Contains every version of every character that appeared before and during Mortal Kombat Trilogy and with their own finishers no less, sans the MK 3 version of Noob Saibot which was a black palette swap of Kano. Basically contains much more content than a real cartridge or the Genesis console can handle. And it can actually run on an (albeit) modded console.
Heboris started off as a simple clone of Tetris: The Grand Master 2 - The Absolute PLUS. Official development of it stagnated in 2002, but 2ch posters decided to take it upon themselves to add many mods to it such as more rotation systems and more modes. These additions were collaborated into Heboris Unofficial Expansion.
Unfortunately, development has stopped because the code—a mix of C++ and a Japanese game script—has turned into a complete mess, and attempts to expand on it have proven futile. For this reason (somewhat), a new Tetris clone, NullpoMino, has been produced, and is much more add-on friendly.
In fact, many mods for NullpoMino, such as the Tetris: The Grand Master ACE modes and the infamous Phantom Mania mode, were incorporated by the author into later releases, with credit given to the original modders.
FCEUX, an NES emulator, features Lua scripting. Lua scripts can directly modify the game's variables. They can also accept user input. The result: Mario, meet Kirby Canvas Curse.
Another example is Neill Corlett's script for Metroid, which adds some elements of Super Metroid, like automapping, and even allows mouse interaction on some screens.
SNES9x, an SNES emulator, also does Lua scripting, as shown here.
Midway produced unofficial sequels of Pac-Man despite only being the distributor of the games for Namco. One of these was Ms. Pac Man, based on a hack of the original game called Crazy Otto, developed by another company with no official connections then to Pac-Man. Incidently, of the Midway Pac-Man games, only Ms. Pac Man was adopted by Namco.
Darius Gaiden got an official (allegedly) hack called Darius Gaiden Extra, which cranks up the autofire rate, rearranges the stages, and adds an option to play all 28 stages, instead of just 7 of them. Strangely for an arcade game, it also adds a pause function.
Star Trek: Bridge Commander starts out with only a few dozen ships of okay quality. Players have added HUNDREDS of ships with quality that makes most older computers cry (due to high numbers of polygons and large textures). There's even new play modes and campaigns, as well as UI and interactivity improvements.
The Glider PRO scenario "SeaCaves" was more than just a custom level; it included a patch for the game program which changed the graphics for just about everything to fit in with its Under the Sea environment. Only the glider itself still looked the same, except for a temporary transformation into a dolphin.
Grand Prix Legends continues to survive thanks to gradual improvements and hundreds of modded tracks and new car sets.
RFactor was built with the intention that as many add-on cars and tracks could be added to it as the community could provide. It's worked. There's (deep breath); Formula One cars, NASCAR cars, Indycars, the old CART-Indy/Champcars, Australian V8 cars, German DTM cars, rally cars... and most major race tracks in the world (and plenty of fictional ones too).
Aquaria has a built-in level editor and a menu option set aside for accessing mods. The modding community doesn't really extend far beyond the game's official forum, but there have been some fairly decent mods created, including a fan-made prequel to the game proper.
Street Fighter II had various hacked bootleg versions in the arcade such as Rainbow Edition and Koryu that modified many of the games properties, allowing for mid air fireballs, dragon punches that spammed fireball and instant character swapping in game.
And for the PC version of Super Street Fighter IV Arcade Edition, we have SSFIVAE Remix, which aims to make the game faster paced...whilst also making most of the moves and characters balls-out insane. Ryu's Metsu Hadouken Ultra, for one thing has been swapped out for a ranbu-esque move. Furthermore, it further differentiates the original Ryu and Ken even more by making Ken a charge motion character.
Ultimate Knight Windom XPlives from this. Being essentially a Gundam VS-based game on its own, it was just a matter of time before packs like SEEDmod, Wind00m and a buttload of various graphic, robot and pilot mods appear. And there's still more to come.
Robot Arena 2 has the DSL: Total Conversion mod, which completely revamps the game, ironing out some bugs, adding all new components to make the game more realistic, and even including replica robots of such famous machines as Biohazard, Razer, Nightmare, Chaos 2 and Hypno-Disc. Said modded version is far more popular than the original version.
Marble Blast Gold has a massive database of custom-made levels, most of which also include custom-made pieces. There are also modifications to the game engine as a whole, most notably Marble Blast Platinum.
M.U.G.E.N was built to be customized, with a highly extensible and completely transparent scripting language. Though released as a fighting game engine, it was originally intended to be a shooting engine, and total conversions have included turning it back into one, among other game types.