Garry's Mod, or GMod for short, is a "sandbox" mod for the first-person shooter computer game Half-Life 2. It allows the player to manipulate objects and experiment with the physics, often with interesting results thanks to the Source engine's physics engine. The construction of complex machines within the game could be said to be a contemporary spin on the Rube Goldberg machine principle.
The current version of Garry's Mod is version 13, which still receives regular updates via Steam. An older, unsupported version, 9.0.4, is available as a free download at download.com among other places. It requires at least one Source engine game in the user's account to play, and therefore is offered in a reduced-price bundle with Counter-Strike: Source or Team Fortress 2 for those who do not meet that criterion.