An amazing paintable physics engine
Like a physics engine inside of a simple version of Photoshop, you can paint with different soft materials (water, viscous materials, etc) as well as rigid materials, fire, and more and then see how they react together inside the paint window.
This is incredibly addictive and fun to show your friends.
Note that this works with the sudden motion sensor in Powerbooks/Macbook Pros when "Motion Sensor" is turned on in the Scene palette. The "drink.oec" file is especially fun.