Here's a fun one from the world of freeware. Sodarace allows you to design your own "robot" to race against the computer's design. The physics in the thing are a blast to watch, and the game plays smoothly. The little java app serves double duty as a design tool, for academics around the world. You have to see it to know what the hell we're talking about. Trust us, it's worth it. It's even worth installing java on your machine, believe it or not.
