For about the last 24 hours I've been trying a new music service (new to the US) called Spotify. It's been in Europe for a while and has garnered a large user base. I must say it has LOTS of tracks. If you can think of a song from your teen years (or earlier), that song is on the list. It looks a lot like itunes, but does so much more.
It's different than Pandora in that if you select an artist, it can play all the music from that artist. It does not just play songs that sound like that artist. A lot of third party sites support the service. For example, www.sharemyplaylists.com will generate a playlist that "sounds like" or is similar to the artist in question, and it provides access to thousands of playlists created by others. Quite remarkable.