Did you ever dream of building your own robot, but you don't have money for parts or just don't want to get dirty with oil and other stuff? No problem! With Golems you can create your own machine and then steer it in full 3d! This sandbox game is free and open-source. If I have to compare it to something, I'd say it's like Algodoo/Phun/Incredibots, but in 3D!
http://www.golemgame.com/
Game requires Java installed.