Pandora is a site in which you indicate a band/artist/song you like. Then, it plays that song, or a song by the band/artist. Right after that one, it plays a song by another artist that has similar features to the last one. Then, you can flag that song as "I Like it", "Don't like it" or just listen/skip it. Meanwhile, it refines it's choices to best serve you
. You can also add other artists/bands/songs, to broaden the site's choices of music it thinks you'll like.
And the best thing is: you don't need to register. It's all free.
I currently have 3 "stations": Electronic, with "influences" of: Tiësto, Kraftwerk, Bond, "Insomnia" by Faithless, "The Killer's Song (Extended Mix)" by Bad Boy Joe and "Hung Up" by Madonna
Hard Rock: Led Zeppelin, MOTÖRHEAD, Dream Theater, Nirvana and Green Day.
Soft Rock: The Bee Gees, Jack Johnson, Donavon Frankenreiter and Coldplay.