Check out this thing called Boffin that Last.fm made.
Last.fm just released an experimental little application called Boffin, which scans your entire music library and creates a tag cloud based on all of the genres in your collection. It doesn't just read the genre tags for every song, though, it instead looks up every song of yours in the Last.fm database and creates a tag cloud using their own tags. So you don't have to worry about retagging all your stuff! The result is something very similar to the regular Last.fm radio, except it plays from your own collection rather than theirs.
Do I want to listen to only folk and punk? Yes please. You can read about it here or download it here for PC or Mac.
It also lets you export your tag cloud to Wordle, which I wrote about before if you remember. This is mine:
PRETTY NIFTY.