In my never-ending quest to keep my ears occupied while I complete the more mundane tasks I have to do at work (like matrix multiplication by hand), I have discovered Turntable.fm. For those who don't know what it is, here's a quick breakdown.
Turntable is organized into rooms. If you don't like any of the rooms, you can create your own. In each of these rooms, there are anywhere from 3-5 DJs and any number of listeners. The DJs rotated selecting the song to be played. In selecting the music to be played, you have access to a huge database of songs already uploaded, or you can upload music from your own computer. A couple of the bands obviously have restrictions on their music. For example, if you want to hear Garth Brooks, you're out of luck. You can hear Garth Brooks Tribute, though, if that's what floats your boat.
Turntable operates under similar restrictions to those of broadcast radio (apparently). This means that they can only play 2 songs from a given artist or album each hour. If you select to play too many, it will skip your turn in the DJ order.
Last but not least, there is a little bit of a "leveling up" effect in the app. Everybody in the room can vote each song as either "Awesome" or "Lame." The DJ who selected the song gets a point for each Awesome vote. No points are lost for Lame votes, fortunately. Different avatars to represent you in the room are available as you accrue more DJ points. Additionally, enough Lame votes in the room will skip the currently playing song.
So that's turntable. It's fun, it's simple, and it's a little addicting. I highly recommend it if you're looking for something different from Pandora. So far there is no mobile app, but supposedly that is under development. Hopefully it will be free, because that would be awesome.