Garry's Mod (officially abbreviated as GMod) is a sandbox physics game using the Source engine. Garry's Mod has been available on Valve's content delivery service, Steam, since November 29, 2006. Despite being a standalone game, Garry's Mod requires 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. 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. Garry's Mod 10 (also known as Garry's Mod 11) is now technically obsolete, as there is now a Beta of Garry's Mod 13.