Tagalicious is a $20 Mac application from the The Little App Factory (best known for their excellent DVD ripper RipIt) which will clean up your iTunes library metadata, fetch artwork, and even find lyrics. So far it's been extremely impressing both in accuracy and price.
I had a "Guns N' Roses" song "1-01 Sweet Child O' Mine.mp3" with existing metadata saying it was the song "Sweet Child O' Mine" from their Greatest Hits album. Tagalicious said it was "Welcome to the Jungle." I played the file in iTunes. Sure enough, my metadata was wrong. It was "Welcome to the Jungle." I have no idea how Tagalicious figured that out, but I suspect it is guilty of practicing
