I've been working on a tagging app for OS X, because I'm incredibly fussy about organising my tunes, thought it might be useful to other folks too.
It gets tags from last.fm for the currently playing track, and then you can choose which ones you want for it, then it tags the track in your last.fm profile. It does some other stuff too, like let you set record labels and instruments for the tracks, and get the lyrics (if you want them).
Anyway, check it out if you like to tag your music:
Mac OS X Music Tagging App