www.Pandora.comCreate a station by suggesting a band, artist, song that you like, or are in the mood to listen to. This site is run by the Music Genome Project(TM). They have analyzed the type of music you suggest and will start playing songs with similar qualities. You can find new bands, new songs, new music, or just sit back and listen to the station play while you work, love, lay out, whatever. As you listen to songs, you can bookmark them, say yay or nay (in which case they discreetly fade out), or buy them from Amazon.com or itunes.com.
You can also listen to other people's radio stations, although I don't think you can tweak them. I created what I called Latin radio (from my suggestions of the Gypsy Kings, Susana Baca, Equatorial by Borges, Chan Chan by Buena Vista Social Club, and Fio Maravilha by Jorge Ben Jor). I also created an easy listening station to listen to while at work. I used Norah Jones, Simon and Garfunkle, Billy Joel, Jack Johnson and Harry Connick Jr. They play some of these artists, but probably 5:1 with new artists in the majority.
Definintely check it out! The free version has ads, but I hardly notice them. They are very minimalistic. www.Pandora.com