I’m a sucker for cool screensavers, and Helios is one of my favorites. Helios combines attraction / repulsion particle effects and smooth surfaces, in what seems like infinite combinations. I especially like that it works with multiple displays, putting something different on each display.
Helios requires OS X 10.3.9+, and is a Universal Binary. Oh, and it’s free.
I enjoyed this one….gets interesting when you combine it with Backgrind (http://desplesda.net/posts/view/9). A little app that sets your screen saver as your desktop.