If you're like me, you've got a healthy collection of mp3's (all legally acquired, of course) that need retagging and you don't want to take the 75 hours out of your life that it would take to do it manually. You should check out this site:
http://musicbrainz.org/They have some pretty cool programs to do the heavy lifting for you. I just tested out Picard and it worked like a charm. You can have it do basic tag correction, or if you want to get real fancy you can use the Analyze tool to have it look at the wave fingerprint of each song and compare it to their database. The second option takes longer, but you don't have to sit there and watch it the whole time. Just fire it up before bed or in the morning when you're leaving for work.