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. Steam Link - £2.99 in the UK, $4.99 in the US. I’ve had my eye on this for a while now, can’t really go wrong for the price.
I have played the "demo" version of this before and had quite alot of fun with it but wasn't willing to pay £9 for it. This seems like a great deal so I'll be getting it ASAP.