Phun is a fantastic free 2D physics simlator designed to bring physics and fun together. It was created with kids in mind but is a good toy for adults as well.

All sorts of things can be created and simulated within Phun. For example you can create a car, wheel tracks and simulate water in a container. It is really easy to create a physical object. By just selecting a brush and then drawing on the screen will make your object come to life. For example if you were to draw a circle in mid air, when you have finished it will fall out of the sky on the default plane.

There are many video tutorials availble to explain various aspects of Phun.