Based on the Music Genome project, Pandora tells genre preconceptions to fuckoff. The high-quality 128Kbps stream serves you an entire station of music that is “genetically” similar to your seed song or artist, sans advertising (other than requisite links to buy the song or album on Amazon and iTunes) .
I have to admit it was a bit disconcerting at first to hear Britney Spears on a Bjork station, but I listened to the musical qualities of the song, and it did actually fit. J-Lo’s “Jenny from the block” that came up two songs later did NOT fit, but that’s why Pandora has made it easy to provide feedback on what songs appear on your stations. Suffice to say, that song will never play on my station again. ^_^
The service is invite only for now, but there’s an invite-request box on the front page (personally, I got my link from the CEO’s post on downloadsquad). Best of all, the preview version is free.