Garry's Mod (officially abbreviated as GMod) is a sandbox physics game using a modified Source engine. Garry's Mod has been available on Valve's content delivery service, Steam, since November 29, 2006. Garry's Mod used to require the user to own at least one game on Steam based on the Source engine, such as Half-Life 2, Counter-Strike: Source, Portal, or Team Fortress 2, although the game no longer requires this as it is its own standalone game now. It is also commonly used for the creation and publishing of videos on YouTube, mainly from Team Fortress 2, Half-Life 2, Portal, Left 4 Dead and Left 4 Dead 2. Garry's Mod was originally a modification for Valve's Half-Life 2, but was later made into a standalone Steam game that was officially released on November 29, 2006 dubbed Garry's Mod 10. The current version, Garry's Mod 13, was released October 24, 2012.