Electric Sheep

http://electricsheep.org/
This is a really interesting screen saver program. It's made up of lots of linked renders called sheep (like the one below), which people can make themselves and submit. They all link up to create a morphing psychedelic visual experience. When it is on it downloads new sheep and circulates them round, with the more popular ones living longer.
Turn the lights out and put on some good ambient/trance and you are sorted.