Physics sandbox game Garry's Mod is now half its normal price on Steam. It'll cost you only $5 to buy this week.

Garry's Mod is not a "sandbox" in the Grand Theft Auto IV sense. There's no actual objectives in the game at all - you're basically just dicking around with the Source engine and its physics. You can summon objects from Source games on your hard drive such as Half-Life 2 and Counter-Strike: Source and...well, do whatever you want with them. The game's very dedicated community has churned out a number of machinimas, maps, and game modes with the Garry's tools.

To play Garry's, you'll need at least one eligible Source engine game installed. The full list of compatible games is here. Also note that the game can be kind of taxing on your system so you'll ideally want about 1 GB of RAM.