Pandora is a free Web 2.0 music sharing service that is just stunning. I think I stumbled on it thanks to a list of the all time most popular del.icio.us sites.
You enter musicians or songs that you like and the service builds a radio station based on your entries. I've been using it for a day and a half now and I have to report that the sound is excellent and the algorithms are pretty dead on for what's been chosen.
My station is KFRK. Share back any stations that you build.