A new version of the MusicBrainz tagger is available for download (win32 version). This version fixes some bugs and adds a bunch of new features (a "saved" tab, ability to have a different filename mask for music by Various Artists...).
MusicBrainz is a tool that allows you to identify and tag mp3 and ogg files based on a generated ID, not relying on existing ID3 tags. So all that music you have that has tags that aren't necissarily accurate, pop them into the mb-tagger and let it go and figure out the correct tags for them, then let it save them into a nice directory structure for you.