Hey there, I just came across http://www.musicbrainz.org/ , and it's great I think. However, their tagger is limited to looking up from and tagging from the musicbrainz database , which makes it very limited .
There is an SDK available, and I personally think it'd be great if Mp3tag supported MusicBrainz lookup. It has a lot of good stuff!
PS: I'd really like optional support for a composer tag too, and keep up the good work! You're doing great so far!