Here's a cool screensaver I've been running for a month or so on my machine, and it is beautiful! It's the Electric Sheep Screensaver, and it's a bit different from your run of the mill savers:

"Electric Sheep is a free, open source screen saver run by thousands of people all over the world. It can be installed on any ordinary PC or Mac. When these computers "sleep", the screen saver comes on and the computers communicate with each other by the Internet to share the work of creating morphing abstract animations known as "sheep". The result is a collective "android dream", an homage to Philip K. Dick's novel Do Androids Dream of Electric Sheep.

Anyone watching one of these computers may vote for their favorite animations using the keyboard. The more popular sheep live longer and reproduce according to a genetic algorithm with mutation and cross-over. Hence the flock evolves to please its global audience. You can also design your own sheep and submit them to the gene pool. "

It requires some large space on your had drive (I have it set to 500MB) so it can download and play those beautiful animations, but it's worth it when you check your emails at night after going to the bathroom :)