UFO: Alien Invasion is a Quake II engine-based open source freeware strategy game that is effectively a direct remake of X-COM, though it adds several new mechanics, changes up many of the old ones, and adds a completely new (and really rather good) story. The current release version as of this writing is 2.5, released in late June 2014.This game contains examples of:
Ace Pilot - Your pilots will eventually become these if you manage them well, although pilot stats have not been implemented so they are equally effective no matter how many kills they've racked up.
Absurdly Sharp Blade - The alien Kerrblade is a dissection tool, while the human-made monomolecular blades are merely terrifyingly sharp and made a little like katana. Both go through flesh like it's butter and Kevlar like it's paper, and generally One-Hit Kill unarmored (or even armored) targets. The Kerrblade does slightly more damage, but as it's not a purpose-built weapon* It's not even a weapon per se; according to the in-game dialogue, it's a vivisection tool. Yes, the invaders are carrying tools to cut you apart and inspect you on the hoof. it's rather bulky and unwieldy. The monomolecular blade doesn't do quite as much damage, but attacks faster, weighs less, and is small enough to be carried in a holster, thus making it a viable backup weapon that effectively obsoletes the standard combat knife.
Alien Autopsy - Performing one allows you to see that species' stats.
Alien Blood - Played straight with Ortnoks which have green blood, but averted with Tamans which have red. The robotic bloodspiders expectedly drip black grease instead.
Anti Matter - Played with: Humans can barely make a few molecules at a time with the contemporary tech, while the aliens seem to be able to fill particle accelerator weapon magazines and spacecraft fuel cans with grams of it.
And while they are at it, they often bunch up together allowing a single soldier to kill a number of them in a single turn, especially if you have explosive weapons or flamethrowers. Aliens also don't know how to pick up items from the ground, and if they run out of reloads, they will just wander around like an idiot unless they have a melee weapon.
Civilians still oddly behave like those in X-Com. They don't try to take cover or run from aliens that are attacking them, even get in your way during the crossfire. They do try to make them smarter recently though.
Most jarring during the mission where you need to protect a military convoy. None of the soldiers are armed and they just run around.
Aliens are still not smart enough to storm your base properly (they will just wander around at the entrance instead of going in for the kill).
BFG - Subverted with the Bolter Rifle, which is huge and bulky because it has to be in order to work properly, not because it is ludicrously powerful. In fact, it isn't significantly more powerful than the standard assault rifle, and actually fires very small rounds (although it has exceptional armor penetration).
Break Out the Museum Piece: Played with. All of the base designs, battle tactics, strategy, and organization are copied right out of the old PHALANX manuals and documents. However all the old equipment is replaced with the latest in 2084 technology. One particular piece of equipment, however, is extremely advanced despite being nearly sixty years old by the time PHALANX gets its hands on it — see the entry for Super Prototype below.
Combat Exclusive Healing - Medikits can only be used in battle, but are capable of healing wounds in a single turn which would require days of hospital time to recover from.
Curb-Stomp Battle: In story the first few attempts to battle the aliens resulted in this... And we were the ones getting stomped. The first battle the soldiers didn't even know what they were fighting before they got wiped out.
Cut And Paste Level Elements - Most of the battle maps tend to rely on pre-rendered sections of terrain being put together in different combinations, just like the original X-COM games.
Death of a Thousand Cuts - If you're not careful, your combat teams will suffer this during periods of heavy alien activity - during particularly hectic periods there can be five or six missions every day, and with no time for your troops to rest and recuperate between missions, all the little wounds add up surprisingly quickly.
Dummied Out - Flashbangs, smoke grenades, and all incendiary weapons (minus the flamethrower) have disappeared from the human arsenal as of the latest release due to bugs. There are plans to restore them, however - the 2.4 dev version brings them back.
Earth-Shattering Kaboom: Well not earth shattering but if the Antimatter Storage ever loses containment, kiss your base and a good dozen miles around it goodbye.
Elite Mooks - Your forces are humanity'sElite Mooks, the best and brightest soldiers culled from the elite elements of every nation's militaries and law enforcement agencies.
Emotion Bomb - Only if you are doing very badly, your soldiers may go berserk and attack each other. This also applies for the enemy that a wave of suppressive fire sometimes make aliens turn on each other.
Note that there's also a small chance low-morale troops will go berserk and take out their rage on the enemy, which usually turns out just as badly for you, because it generally results in them charging blindly into enemy fire. It's quite Badass if they survive, though.
It's also possible for friendly or hostile soldiers to freeze up completely if their morale is low, cowering uselessly behind the nearest piece of cover.
Frickin' Laser Beams - Played about as accurately as man-portable lasers can be—they are extremely accurate but have relatively low damage and VERY low stopping power, and their beams can be scattered by smoke, fire and dust.
Ditto for Particle Beam, except it is substantially more powerful, but slightly less accurate and subject to same penalty to fire and smoke.
Guns Akimbo, Dual Wielding, or Sword and Gun - Can be done with some accuracy penalty. You can also dual wield a two-handed weapon with a handgun, but you cannot fire the two-handed weapon without the other hand free. Even if you only use pistols, you are still only able to fire the weapons one at a time. The only advantage of wielding two weapons is that you don't have to pull out the other weapon from your holster when you need it, which cost some TU (just think of the advantage of having a handgun in one hand and a Kerrblade or stun rod on the other).
Imperial Stormtrooper Marksmanship Academy - Fortunately, it isn't quite at X-COM levels, and even the worst recruits can at least hit what they're aiming at occasionally. That said, both your troops and the aliens can be wildly inaccurate at times, even at point-blank ranges. Crouching before firing largely negates this issue, though.
Infinity Plus One Gun - The Coilgun in 2.3, though you need to obtain an alien needler in order to get it.
Instant Home Delivery - You can sell equipment at one base and immediately buy it back at another for the same price. What makes this especially egregious is the fact that there's also a "Transfer" option that allows you to send equipment between bases, but this generally takes a few days, whereas selling-and-rebuying happens instantly.* This has since been mitigated by introducing a slight price discrepancy between buying and selling, but the problem of instant delivery remains. New recruits (Scientists, Workers, and in recent build Pilots too, not just soldiers) are delivered instantly as well.
Kill It with Fire - Flamethrowers are deadly against most aliens since their armors, while good against bullets can barely protect them from the flames. It is also one of the two starting weapons, the other being Grenade Launcher, that continue to be useful until the end. The only weaknesses are limited range, heavy weight (which precludes weaker soldiers from using it without a heavy speed penalty), and its rather bulky ammunition.
Kinetic Weapons Are Just Better - Played with. Although bullets are quite good against unprotected alien flesh, alien armor does a disturbingly good job protecting its wearer against gunpowder-accelerated projectiles, and as such by the mid-game the vast majority of the standard human arsenal will be obsolete. The grenade launcher and the flamethrower remain useful for the entire game, as the former can be adapted to fire a cartridge version of the alien plasma grenades and the latter is a cheap way to light things on fire, but they aren't technically firearms.
Leeroy Jenkins - Some alien crafts appear to behave this way in recent builds, charging blindly towards vastly superior enemy forces rather than running away.
Magnetic Weapons - The Bolter rifle, the alien Needler, and the Coilgun from the v2.3 release.
Mohs Scale of Sci-Fi Hardness - Pretty high up as far as military sci-fi goes. The game's built-in UFOpaedia articles have fairly lengthy explanations of the tech, and the technology includes everything from antimatter engines and buckyball-infused armor to accurately-portrayed lasers and caseless ballistic weapons.
The detail it goes into each weapon, vehicle, alien or even ammo type puts the Codex in Mass Effect to shame.
More Dakka - The human grenade launcher has a burst fire setting. Also, the plasma blaster is powerful and rapid-fire, but has terrible accuracy, putting it squarely under this trope.
The Needler is pretty much the epitome of this. Instead of normal sniper weapon which fire single powerful slugs, this one actually go the other way around by spamming hundreds of small needles instead.
The human machine gun is, obviously, rather good at this, making it an extremely useful suppression weapon in the early to mid game.
Multinational Team - The game's code even tracks them by nationality, although there's no indicator of country of origin in the interface yet.
Obvious Beta: Justified because, well, it's still a beta. Remember, save early, save often!
Oddly Small Organization: Even if you're doing exceptionally well, PHALANX will rarely have over a hundred combat-ready soldiers, with 40 or fewer troopers being the typical amount. You'll usually have a far larger number of scientists, workers, support staff, administrative personnel and pilots, but the organization is still extremely small considering its area of responsibility is literally the entire planet.
Oh, Crap: Ignoring the amount of times this happens to you while playing, reading the research letters has the Chief Science Officer having a lot of these. The most notable example is where they are taking a UFO apart and find that it's powered by antimatter—and they only found this out after opening up the fuel tank.
Also several times he will also realize (again) just how technologically advanced they are compared to us.
Pistol-Whipping - You can bludgeon aliens with your guns in recent 2.3 updates, though proper melee weapons will do a better job.
Relationship Values - For each of the multinational power blocs funding your organization. Their happiness with you influences the amount of money you receive and the number and quality of recruits you're given - a nation that barely tolerates you will give you barely any money, and either very crappy troops or no troops at all, while a nation that loves you will shower you in money and give you the best and brightest men they can find.
RPG Elements - Your soldiers have a series of stats and skills that improve with use.
Save Scumming: Not a completely straight example since saving and loading isn't allowed in battle. However you can hit the reset mission button as many times as you like.
Scifi Writers Have No Sense Of Scale - The original story for UFO:AI had the aliens invade earth on their mothership, housing its entire population (several trillion aliens). In order for the mothership to contain, let alone sustain, those numbers, the ship would have to be larger than Jupiter.
Averted later with the antimatter specs. The chief science officer says at one point about current antimatter production on Earth "That may sound like a lot, Commander, but it would take us 80 million years to make a gram of antimatter at the current rates."
Short-Range Long-Range Weapon - The sniper rifle from earlier build appears to be unusually inaccurate even if it is used in what is normally Assault Rifle range. Fixed in newer build. Laser weapons are exact opposite as even the pistols are deadly accurate at long range, but they don't do much damage to compensate it.
Short Range Shotgun - Largely averted. The riot shotgun has decent range—if not necessarily great stopping power at maximum range—with flechette shells, and can alternately be loaded with slug rounds. The micro shotgun is a bit truer to this trope, though.
Shown Their Work - All over the place. It helps that anything wrong will quickly be pointed out in the forums.
Shout-Out - The Plasma Pistol looks a LOT like Halo's plasma rifle.
Standard FPS Guns - This is not an FPS game, but the weapons appear to have this idea, one for each family of weapons. It's actually justified in-story - a program called 'Excalibur' was instituted that selected a range of different weapons from throughout the world in various different classes, collecting the best weapons humanity has to offer and providing them to your troops in a standardized form.
Standard Status Effects - What the flashbang does, the aliens don't have them since they don't care about stunning or collateral damage anyway.
Of course, they'd do a lot better if they did - the flashbang is arguably one of the most useful tools in the human arsenal, and skilled deployment of them makes virtually any mission a cakewalk.
Super Prototype - The UFOpedia file on the Stiletto-class interceptor has Cdr. Paul Navarre expressing his shock at how capable the aforementioned craft is, and disbelief that it was never adopted by the us military, being far more maneuverable than any helicopter, and possessing the speed to keep up with supersonic fighter craft. To be fair, its "tricky" handling characteristics would have made it unsuitable for the average pilot.
Universal Ammunition - The DF cartridge used by all laser weapons, including the electrolaser. Realistically, bigger laser weapons draw more power per shot and thus ensure the DF cartridges provide "less" ammunition per reload for bigger weapons.
Video Game Flamethrowers Suck - Averted. The flamethrower is one of the best close-quarters weapons in the game, remaining useful in a room-clearing role well into the late game, and the other incendiary weapons were equally effective before they were abruptly and inexplicably removed from the game.
Wave Motion Gun - Particle Beam Cannon, especially the ship version. To lesser extent, heavy lasers.
Wave-Motion Tuning Fork - Plasma weapons appear to work on this principle based on their shapes, but reading the in-game UFOpaedia entries on them reveals that the plasma itself is actually generated internally, and the 'tuning fork' assembly is actually a pair of spinnerets which generate a thin film of an advanced plastic-like material to (briefly) contain the plasma, thus forming it into a projectile.
We Have Reserves - It is possible to play like this, but unlike in X-COM, this strategy is strongly discouraged in this game due to the limited number of recruits available per month (especially on harder difficulty) and the bad starting stats of new recruits. You have to train them and keep them alive.