Garry's Mod is an independent mod of the Valve game Half-Life 2 created by Garry Newman. Originally just Half-Life 2 with the rope, manhack and rollermine guns, it has evolved considerably since then. Rather than taking place in any sort of universe, the game is a sandbox where you can construct and demolish buildings using props from various Source games like Counter-Strike, Team Fortress 2, and Zeno Clash. The ragdolls and even NPCs from these games are also at your disposal. Considered one of the most popular machinimadevices as of now. The game uses a lua scripting format for its weapons and events, meaning it's possible to code your own weapons into the game. Currently, it's available on Valve's Steam client for download.
This sandbox provides examples of:
Abnormal Ammo: Very frequent. One of the sample scripted weapons (or SWEPs in community jargon) included with the game, the Flechette Gun, is exactly like the USP from Half-Life 2 only it shoots an endless stream of the exploding rounds fired by the Hunter enemy from Half-Life 2: Episode 2. You can build prop cannons that can shoot everything from sawblades to watermelons. And let's not get started on the various SWEPs devised by the community...
The eponymous creator of the game, once scripted an AK-47 SWEP that shot babies as ammunition. In response, someone created a Baby SWEP that fired AK-47s, and another person made another AK-47 SWEP that could shoot either babies or sawblades.
A mod of the already awesome molten rebar crossbow made it shoot sawblades.
One SWEP, named the "Scavenger Cannon", is, essentially, the Rock-It Launcher - suck up any prop you want with it and then launch it, sort of like a rapid-fire gravgun.
Artificial Stupidity: Most spawned NPCs will stand ramrod still, firing wildly at anything that comes nearby, unless the map they are spawned on is "noded" properly to allow for complex AI.
Or if the user downloads one of the several mods that allow manipulation of NPC behavior. Can be useful for Machinima.
Bare Your Midriff: Nahka's ports of his Oblivion characters all have outfits like this.
Beam Spam: The GCombat and Spacebuild Enhancement Project addons present many different ways to do this.
If you downloaded GmDoom and mounted the correct .wad file then you can have 200 of either of them, depending if you picked up a soulsphere, mega armor or a megasphere. Due to Source's method of handling armor points, you don't get the 3/4 damage reduction like actual Doom.
Humanoid Abomination: A lot of them. Special mention goes to the Vagineer, the Engina, Dic Soupcan, and many more.
Loads and Loads of Loading: Downloading excessive numbers of addons can result in very long load times. Garry did an automatic survey of GMOD users, and there was one person who had an average loading time of over 25 minutes.
Especially during the short time that the game had a strange error which multiplied load times by a factor of about five. 15 minutes just to get on to a regular server. If you got kicked, it might as well have been a fifteen-minute ban from every server in the game.
Le Parkour: Possible using the Parkour Swep (a scripted weapon which allows you to jump large distances and negates falling damage), Perfected Climb Swep (a scripted weapon that allows you to climb and grab onto ledges), Smod Leg (The glorified kick from SMOD), Gordon's Fists, and a big, urban themed map (oh let's say GM_Bigcity or Gm_madness~city)
Massive Multiplayer Crossover: There are a lot of models available for this game. Many of them are ragdolls of characters from other games, which can be used to create some rather interesting scenes if you're good at posing.
This is because most models need only be hexed and then imported.
Nintendo Hard: Posing is rather tricky, even if you know what you're doing.
Large, gangly ragdolls can be a nightmare, and can potentially kill you if you're too close while the physics causes them to flail about because you moved their arm a pixel in the wrong direction. The author of Concerned once got killed while posing a Strider, as the Physics Gun jerked the Strider's leg too fast at him. Can be offset by placing heavy objects near them, to help control their jerkiness. Possibly-relatedly, he only had a posed Strider show up in one other comic, nearly two years later.
Finger slipped from E to R when pointing at the ground with the Physics Gun out? Say bye to your contraptions!
Noobs: Called "Mingebags" by the community, after the default player name before the game was updated to use your Steam screen name as your player name.
Nothing Is Scarier: As you'll often be The Aloner (unless you're in Multiplayer, of course), this can kick in on night-time maps, or just in general. Also how most horror maps do their business, since the closest thing to getting scary with Mooks are the regular Headcrab Zombies (packaged with G Mod), and those often get boring fast. Combined with Hell Is That Noise, maps like Gm_Ghosthunt and Gm_Scream have terrified audiences without even showing an enemy.
Not Quite Flight: Balloons+thrusters, typically combined with the airboat. It is possible to ride sufficiently large props via phys gun or thruster use, but good luck getting down (alive anyways).
Percussive Maintenance: The easiest way to fix a spazzing contraption (when the physics engine breaks and sends everything spinning around wildly out of control) is to pull out the Physgun, grab the contraption, and repeatedly smash it against the nearest wall until it stops spazzing. Unfortunately, delicate player-made mechanical objects (such as guns and engines) require much more finesse to fix - performing Percussive Maintenance on these will usually make the problem worse.
Player Versus Player: Straight Deathmatch servers exist, however, there are many "build to fight" servers, where players build contraptions to fight each other with. Like weaponized windmills.
Ragdoll Physics: Most Garry's Mod Machinima depends on it; There are maps and gamemodes dedicated to slaughtering ragdolls in the most gruesome ways imaginable.
Rule of Fun: Oh so much. Highlights include building random things out of props, destroying them, making NPC's fighting each other, destroying them, blowing up stuff with nukes, downloading addons, destroying them, etc.
Rummage Fail: As mentioned above, you may find yourself doing this after an extended session of messing around with SWEPs. It doesn't help that only a few user-created ones can be dropped and the only way to reset your arsenal without losing your Half-Life 2 weapons, physgun, and toolgun is dying, leading to a rather odd example of Death by Materialism.
Run Don't Walk: In the default controls, the Walk function isn't even macroed to a key (back when it was, the corresponding function didn't even exist). Although, running is faster.
If players want to move slowly, they generally crouch anyway. The only difference between that and walking is the animation and the smaller hitbox. And the fact that it's bound by default.
The only problem with crouching is that it changes your viewpoint (I.E. where you're pointing your physgun), meaning that if your posing a spazzy ragdoll or prop like a Strider or a Sentry Gun, you risk it spazzing out from the sudden motion; most players avoid this by freezing whatever they're posing, then crouching.
Self-Deprecation: The Steam Trading Card pack for Gmod has the "Missing Texture" card, showing the infamous purple-and-black checkerboard that plagues so many custom maps and models. "Invasion" shows the Mingebag - the unanimated default model from Gmod 7 and prior versions; Kleiner with his arms sticking straight out, a physgun in his crotch and his legs ramrod straight.
Some of the badge levels from the game as well such as Spammer and Crasher.
Skybox: Most decent maps will have one of these. The really good maps will have a detailed, 3D skybox. You can even put props and ragdolls inside them to make them appear massive. The only drawback is that they do get confusing on maps like the new flatgrass, nothing is more startling then crashing into the Invisible Wall in your helicopter, plane, car or whatever vehicle you built.
Super Not Drowning Skills: You can stay underwater as long as you like with no ill effects, due to GMod making your suit energy infinite.
Wimp Fight: The most effort ever put into fight scenes in comedies usually involve welding invisible thrusters to the characters' limbs and holding down the ignition button. Justified, as it's meant to be funny, not serious, and it is indeed funny.
One of the best examples is Das Bo Schitt's "Billy Mays Vs. Vince Offer". It's hilariously epic due to the incredibly over-the-top slap-fights and body slams.
Wreaking Havok: Find GCombat, or Garry's Bombs, or any addon involving large amounts of explosives, and go wild with them.