I noticed last.fm a few days ago via Joe Thorn. Now Joe has a post up on it. It's a free music player that plays according to your musical preferences. So for example, I tell it I like Rogue Wave, Wilco, and The Avett Brothers and it plays music from these guys as well as from groups and singers who are in the genre ranges of the ones I like. Pretty cool.
You can skip songs you don't like (even block them so they won't be played again) and mark the ones you really like. There is much more to the site than this, but this should be reason enough to check it out.