This is a really snazzy website. What it does is it looks at your Itunes library, takes sections form songs and joins them to prouduce a summary of your music collection. It's rather snazzy and works out of your browser. I ran it in IE, haven't tried it in anything else. It's very configurable so you should be able to produce a signature that sounds the way you want it.
Anyway, if you've got Itunes and a lot of music, give it a shot. It's only failing is that it can't work through the DRM on Itunes Music Store music, so if you've got a lot of purchase music, it'll just ignore it. However, there are ways to strip the DRM off the songs if you're interested.