Steam - 'Garry's Mod' Downloadable Game - $5USD
"Garrys Mod is a physics sandbox. Unlike regular games there arent any predefined aims or goals. We give you the tools and leave you to play.
You spawn objects and weld them together to create your own contraptions - whether thats a car, a rocket, a catapult or something that doesnt have a name yet - thats up to you."
*This game requires that you own a Source engine game.