http://www.itunesregistry.com/register.php

This site allows its users to upload their iTunes playlists and run them through a (growing) series of reporting tools in an effort to help you, the listener, better understand your musical collection.

Some of the interesting things available (once you make an account) include a Browse-By-Letter tool for your iPod, rankings of entire songs and albums, an estimate of how diverse your music collection is and a handy little report that sits in your iPod’s ‘Notes’ folder, and gives you some interesting statistics on each of your genres.