writes "The Music Genome Project, an attempt to define music by it's traits in a way similar to DNA defines traits in humans has led to the development of Pandora. Pandora uses the song choices you make to see what traits appeal to you and present you with custom radio station. While limiting you to thumbs up or thumbs down, the "gene" heuristics allows for a very quick adaptation to your musical tastes."
Not sure how deep it goes, and I'm not sure I like that it led me from The Who to Styx and Def Leppard. But this is a neat little tool for discovering new music.