Via the Win Tech Off Topic mailing list, I learned about Pandora earlier today. It's an outgrowth of the Music Genome Project.

You create stations in Pandora by telling it an artist or song that you like. It starts playing music that it thinks you will like, based on its reasonably extensive database of carefully characterized music. You can tell it if you particularly like or dislike its selections, to guide its future offerings on that station. You can have up to 100 personalized stations.

So far, it's doing a good job.