Planetary is a free iPad app that provides an entertaining visualization of your music library. The music on your iPad is displayed as a 3D universe where artists are stars (of course), their albums are planets and songs from the album are moons. The ability to rotate the 3D view of your library and fly through it is a nice visual experience and very entertaining.
However, Planetary presents sections of your music universe simply sorted alphabetically. So, while it is visually pleasing, it cannot guide you in song selection. It would be much more interesting if it were able to visualize relationships between songs using data found in iTunes Genius, Pandora, or the Moodagent app for the iPhone. This would allow you to discover relationships among the songs in your collection in a much more interesting and useful way.