http://www.pandora.com/ (It's free...) "The Musical Genome Project" Pick a song and or singer, and it will play that song or a song by that singer, plus others with similar styles and influences. Tell it a list of who and what you like, and it will create a station. Only problme is, other than fast forwarding through songs your don't like, you get not pick of the songs - but it's pretty good itself of guessing what you might like. http://www.pandora.com/
I've been a Pandora listener for about a year now.... I just recently added it to my iPhone!! The only drawback is that I tend to be into film scores nowadays and Padora's offereing in that selection seemsto be a little sparse. John williams is great and probably the best, but there are a lot more talented compsers out there.
Pandora seems pretty cool so far. I have two channel sets up so far, "Danzig" and "Pink Floyd". I'm still messing with it.
I think my tastes are far too eclectic for Pandora to work well without making several channels for each particular style, which doesn't really match the variety of genres I listen to when gaming or whatever. Seems like every time I make a new station, it eventually ends up turning into a playlist of shit I can't skip due to licensing.