Garry's Mod

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 Machinima devices 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.
 * Beam Spam: The GCombat and Spacebuild Enhancement Project addons present many different ways to do this.
 * BFG: The original has been made into a scripted weapon, complete with the original sprite as the viewmodel.
 * Camera Abuse: The Camera tool is very easy to knock out of place if it's not welded to something.
 * Getting slapped by NPCs or falling objects while wielding the Camera gun also counts.
 * Character Customization: Most RP servers have this, in some form or another and to varied extents.
 * Exploding Barrels: You'll have so much fun with these. So much fun...
 * By "fun", we mean "let's play crash the server by spawning enough of them to fill the map and then detonate one".
 * First Person Snapshooter: The Camera allows you to take pictures from in-game and have them saved to file. Hence, there have been many Web Comics created via Garry's Mod.
 * Game Mod: Not only is the game itself this, but it has another built in one where you race watermelons.
 * And build tanks.
 * And build spaceships.
 * And fight zombies.
 * And Perform Le Parkour.
 * And chase/get chased by the Ultimate Chimera.
 * And play tower defense.
 * And play fort wars.
 * And play RTS as a god.
 * And play RTS with watermelons as the units.
 * And find out who the traitor is.
 * And get stuck on an island.
 * And create a boat to survive a flood.
 * And disguise yourself as inanimate objects for a game of Hide and Seek.
 * And film movies.
 * And ride an elevator.
 * Griefer: So very much. Much like the "Noob" example below, they are called Mingebags. Examples include:
 * Killing while building
 * Killing with build objects
 * Killing with coded objects
 * Killing... Well, there are many ways.
 * Crashing the server by Wreaking Havok a little too much.
 * Playing sound files -- especially low quality ones -- without anyone's consent (or approval)
 * Blocking off the spawn so that no one can get out.
 * Hit Points
 * Hyperspace Arsenal: You can keep all of the weapons in the "weapons" section of the spawn menu on your person, which can lead to a Rummage Fail. Better yet, you can't drop weapons.
 * Improbable Weapon User: You can kill people not only with guns, but also with their ammo pickups.
 * Infinite Flashlight: Compared to the Ten Second Flashlight one of the original games.
 * Intercontinuity Crossover: Various mods allow players to import characters from other non-Valve franchises, from Super Mario Bros to Sonic the Hedgehog to My Little Pony: Friendship is Magic, Star Wars, The Legend of Zelda, Resident Evil, Mortal Kombat, etc.
 * Joke Character: The Hobo from Dark RP. No salary, looks like a corpse, and only has the ability to throw crap at people (literally).
 * 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)
 * Ludicrous Gibs: The Dismemberment Mod.
 * The Gore Mod probably takes home the gold, though.
 * Machinima: Tons of it and then some.
 * Concerned was made using almost solely Garry's Mod.
 * The G Mod Idiot Box, courtesy of Das Bo Schitt.
 * The video versions of Half Life Full Life Consequences, in all of its insane glory.
 * Just search "GMod" on YouTube, you'll get thousands of results.
 * Magnet Hands: It doesn't matter if you're underwater, in outer space, on fire, flying at 200/kph, you'll be holding your tool gun/Gravity Gun/Physgun in an indestructible death grip.
 * Until Critical Existence Failure hits, and then the weapon will disappear entirely.
 * The Many Deaths Of You
 * 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.
 * More Dakka: The Turret tool, among other things.
 * Muzzle Flashlight
 * 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.
 * 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).
 * Nuke Em: Garry's Bombs, and Teta_Bonita's nuke addon (which allows you to choose your preferred method of detonation).
 * Offscreen Teleportation: The Harmless Companion Cube addon. Just don't forget to delete it before you turn away from it. Whatever you do... Do NOT freeze it. That just makes it stronger...
 * The toybox now has Weeping Angels.
 * 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... Did I mention Destroying Things just for the hell of it?
 * 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.
 * Science Fiction: The Spacebuild gamemode, usually used in combination with the futuristic props of the Spacebuild Enhancement Project. Not that's there anything stopping you from just sticking a oxygen tank on a WW2 fighter and flying off to Neptune, though.
 * Skybox: Most decent maps will have one of these. The really good maps will have a detailed, 3D skybox.
 * Surreal Humor: Garry's Mod is frequently used as a medium for this. Google "Rubber Fruit", for instance.
 * Elevator: Source derives half of it's humor from the fact that you will more often than not find a beach and a detective's office within two floors of eachother. Inside an apartment complex.
 * Tank Goodness: The premise of the ACF (Armored Combat Framework) addon. Players build tanks, add armor panels, stick in ammo caches, give them guns and then duke it out.
 * The Many Deaths of You: The aforementioned Ragdoll Slaughter, you yourself can ragdoll upon death, so...
 * Throw It In: At one point, Father Grigori was mistakenly put in the "animals" NPC section, but Garry found it amusing due to just how accurate that was, and let Grigori stay in the animals section.
 * Wide Open Sandbox: You're not going to run out of stuff to do.
 * Wimp Fight: Most effort ever put into fight scenes in comedies usually involves welding invisible thrusters to the characters' limbs and holding down the ignition button.
 * For good reason, 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.
 * Hell, all you need is wire mod really, with the explosives you can have a lot of fun figuring out how to make you own Nukes.
 * Another good one is the Stargate mod and it's many addons, hell with the Stargate, just standing too close to the gate will cause vaporization like in the show, hell another superbomb example is the Naquadah Bomb which is similar to the fusion bombs you can make in Spacebuild (don't ever make one intentionally on a server, it's pisses people off, and will often get you banned fast (also, it's the main reason I play on SB servers that use the Stargate mod (like most), so I can just use a ZPM instead of a fusion reactor)).