garry

Garry's Mod is a sandbox mod for the Source Engine. Unlike normal games there aren't any predefined aims or goals. Players are given tools and are left to entertain themselves.

The player is in a world where objects can be created and destroyed on demand. Where people can be created and destroyed with the click of a mouse. A world made for him. A world he made.

Garry's Mod gives the ability to program your own gamemodes (Don't worry - you don't need to be a programmer to play them). This allows the community to make new, often wacky, gamemodes such as melon racing, build to the top and so on.

It's often worth taking a look in the server list to see if any of the games prick your attention - there's often something weird there to try out.
Garry's Mod benefits greatly from a thriving community which is constantly churning out new ideas, maps, models, gamemodes, weapons, Lua scripts and skins.