Electric Sheep is a free screensaver that lets your computer join a global network of computers processing "electric sheep"--striking, ever-changing, abstract fractal animations generated with a genetic algorithm (taking their name from the novel "Do Androids Dream of Electric Sheep?" by Philip K. Dick).
Your computer downloads and displays sheep from the current "flock" as your screensaver, while running in the background, your CPU contributes to the parallel computation and mutation of new sheep, joining thousands of other computers around the world. You can also vote on sheep with your up and down arrow keys to help guide how new flocks evolve. Electric Sheep now also has better compatibility with Snow Leopard, including support for multiple monitors--and you can set them up to render different sheep or the same sheep.
Electric Sheep works best with a broadband connection, and casual users might be frustrated by the app's setup, since it can take a long time to download the initial flock. (More sophisticated users can kick-start the process by manually downloading the current flock with BitTorrent--which unfortunately isn't integrated into the Mac client for Electric Sheep.)