Pandora - Your own radio station
This is very very cool.
They've built a 'genome map' for music. You write an artist/song name that you like and they build a 'Radio Station' for you.
10 first hours free, then it's $ 36 per year.
The neat thing is that it actually works! I liked most of the songs it played in my 'Pavement' radio. My 'Belle and Sebastian' was not that good but I'll keep trying ;).
Yahoo has LaunchCast radio that it's tries to do something similar. You need to select a set of artists you want and then it plays the music that is popular among the people who like those artists. I tried it, and I got bored after the 3 first songs.