Warning! This OP is very stuffty.
http://www.golemgame.com/Golems is a 3D recreational physics simulatorBuild robots, spaceships, clocks, catapults or cannons.
Add sensors, timers, switches, motors. Give it a brain. Make a mouse trap - or make a mouse.
Take the wheel and pilot your machine yourself.
Build anything.
Rube Goldberg Machine
http://www.youtube.com/watch?v=SrW-nT3j4nwHere's a rear differential that I made and I'm quite proud of.