Mac OS X users should take a look at breve. Breve is a rather interesting 3D simulator, that includes a screen saver module in addition to some other things. From the productis Web site:
breve is a 3D simulation environment designed for simulation of decentralized systems and artificial life. While the concept is similar to existing packages such as Swarm and StarLogo, the implementation of breve--which simulates both continuous time and continuous 3D space--is quite different. breve also includes an experimental physics engine allowing the user to implement real-time physical simulations.
In other words, breve can simulate things like a creature that learns to walk on your screen, or a swarm of bug-like dots.
Even if you donit have the time to learn how to program your own simulations it might still be entertaining to give the screen savers a try, which run these impressive looking simulations during your idle time. You can find more information about breve at the productis Web site.