A sandbox game where you can use stuff from any of these games (if you own them): Half Life 2, Counter-Strike: Source, Portal and TF2.
You need at least one of these games to be able to run it. It is also Multiplayer.
If you want to see what you can do on Garry's mod, look up "The Idiots of Garry's Mod 2" (the first one isn't as good) The video is really funny and shows lots of stuff on Garry's Mod.