Garry's Mod is a purchasable standalone mod for the source engine, mainly HL2. It uses the models,sounds and levels of any source game that you have installed along with millions of downloadable content pieces from garrysmod.org . However you must own at least 1 game from the orange box for it to function. This game is all about having a laugh. There are dozens of different game modes ranging from complete sandbox to parkour to garry's theft auto to hoverboard races. Garry's mod allows you to build and create whatever you want, however you want it and wherever you want it. This game is not for somebody without an imagination though so if your not into sandbox style games or building and creating contraptions stay clear of it. Some of the most fun I have had in the game though is playing through the regular Half Life 2 maps with a couple of other people and building some crazy flying death mobiles and racing through the maps.