Electric Sheep

is a wonderful screensaver. This description is shamefully copy-pasted from the website, www.electricsheep.org:

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.

I've been using this for a few days and it really is fun. Be warned though that you'll need broadband (or a 100kbit down, 100kbit up) connection to download and upload the Sheep. Otherwise, it's a great screensaver that looks very geeky. Exactly what we geeks like.