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Garry’s Mod is on sale for 75% off as the Steam Daily Deal for 11/2/2012.
Garry’s Mod = $2.49 / £1.49 / €2.49
This is a sandbox standalone game that has a lots of game play mods of its own. One popular recommendation is Trouble in Terrorist Town. Keep in mind that you might be missing some texture/models if you don’t own Counter-strike: Source. I think it’s funny that the developer is just listed as Garry.