Garry's Mod is a sandbox game built off of Half-Life 2. It allows the community to create or import any items, vehicles or weapons imaginable and put them together in one server.
Developer(s): Facepunch Studios
Only on PC
Garry's Mod, often nicknamed "G-Mod" is a sandbox that gives players the basic tools to come up with your very own creations and share them with people worldwide. It is extremely reminiscent of Valve's Half-Life 2, mainly because it runs entirely off of the game's engine (and also uses the same default HUD).
The game launched on December 24th 2004, with the Steam launch arriving on November 29th 2006. The newest version, dubbed Garry's Mod 13, was released on October 24th 2012.