Audioscrobbler is a cool idea
. It's described as an "exploration into Social Information Filtering" by a CS student at the University of Southampton in the UK.
Audioscrobbler is a plugin for Winamp or XMMS that records what songs you listen to and then recommends music based on other people who listen to the same types of music that you listen to. Cool idea, works with native applications (no Yahoo-type Launch player to download)... the next logical step would be to either offer the recommended songs for download or streaming, or barring that, offer the songs for purchase through an online retailer.
I'm waiting for the Winamp3 plugin to come out, and I'm curious as to how well the music recommendation algorithm works.
FYI, as I'm writing this, Radiohead is the most-played artist.