Garry's Mod is $5US = $5.80AUD this week on Steam.
Game description:
Garry's Mod is a physics sandbox. Unlike regular games there aren't 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 that's a car, a rocket, a catapult or something that doesn't have a name yet - that's up to you.
If you're not too great at construction - don't worry! You can place a variety of characters in silly positions.
Great game if you have an imagination ;]