The physics sandbox Garry's Mod has just been updated to include Steam achievements and is now 50% Off until September 21st. Join in the fun today!
This game requires that you own a Source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source. Click here for a full list of games that meet this requirement. Please note that Portal: First Slice and the bundled Half-Life 2: Deathmatch do not fulfill this requirement.