iEatBrainz: MusicBrainz for iTunes
iEatBrainz is an alternative to the MusicBrainzTagger developed by Jay Tuley for MacOS X users who use iTunes to manage their audio files. It works together with iTunes to retag your files according to the information in the MusicBrainz database.
If you don't use iTunes, you may prefer to try the PicardTagger, which has some advanced features that iEatBrainz does not (however, the MacOS X support may not be complete, so it may be less stable). If you use iTunes, iEatBrainz is probably a better choice, since any application that changes the tags on files directly may confuse iTunes, which keeps its own cache of that information.
However, iTunes 4.7 (and probably some earlier versions too) does check to see whether a file's tags need to be reloaded whenever it is is selected in a playlist or played, so this is probably not of concern anymore. The worst case scenario would occur when the tags for one or more of the spelling of the artist, album, song, disc or track number is edited in an external editor, iTunes will notice the changes, and still be able to find the file, but it might not automatically rename / move the file until the next time one of the aforementioned tags is edited within iTunes itself. (Furthermore, this only applies when iTunes has been configured to "Keep iTunes Music folder organized," under "Advanced" preferences).