You gotta try Garry's mod for Half-Life 2

If you did not try Garry's mod yet, you absolutely must try it, you can get it from:

http://www.garry.tv/garrysmod/

Its amazing the number of things you can create, I spent the whole last weekend with my son building new inventions, you can really just build new machines and the physics just works, amazing. Our best invention was a space shuttle with thrusters on every side, we can control the thrusters with the number pad, we then jumped in and flew together (you can have more than one person in the same world, and can play it online too). Our stupidest invention was a car made out of a human, we split his/her hands and legs open, put wheels at the end of them, and stuck a thruster in the butt, it was hilarious ;)