If you haven't tried it, Turntable.fm is a multiplatform music-streaming service that's made up of different user-created "listening rooms." You can wander through rooms and soak up the different genres, create your own rooms, or even take a turn spinning your own playlists (via MediaNet's catalog) while other listeners vote (up or down) on your song choices. Turntable.fm offers a completely unique way of discovering music, different from Pandora's or Grooveshark's.
To get started, create an account, and start browsing through listening rooms. On the main menu screen, you can search for a room by name, or browse through popular rooms, favorites, or only those with empty DJ seats. Before entering, you can see how many listeners are in each room, but you can't see a full room description. This makes it difficult to quickly browse, as you actually have to enter rooms to get a feel for their music.