Planetary is an iPad app designed to allow you to explore your music library as a planetary system. It has a very pleasant UI, but perhaps what is more interesting, is the taxonomy used to explore your library. Artists are stars, their albums are planets orbiting stars, and tracks are moons. The moon orbit planets at the speed of the track and their size correspond to play counts.
The orbit speed may be a little gimmicky, but the rest is a coherent way to browse music. There are options to sort music if you have a large library and the Cinder C++ library helps the program to be snappier, despite the heavy 3-D aspect.