Welcome to MusicBrainz!

Musicbrainz is one of those handy applications that combines with a website to make it a vital part of your mp3 collection. What does it do? What doesn't it do? HA! (Ok enough with the cheesy sales pitch)...

Basically, it runs the song through a program to come up with some "TRM fingerprint"... then it checks those fingerprints against the database to ensure that your mp3 is properly tagged.

You can always update the database if the song you ran through it wasn't already there, and although they are almost as bad as wikipedia about rejecting submitted information, it's better than nothing. Just remember that without the tagger, this site isn't terribly useful, though you could use it passively to look through an artists discography and see what album it was, when it was released etc.