MusicBrainz is an open source music database that serves a variety of useful purposes to music lovers. The database is an extensive collection of music information organized by artist relationships, but can be sorted and accessed by a variety of means. The growing community has helped the MusicBrainz project evolve rapidly since its inception, boasting a database of 175,037 artists at last count. While its numbers may not be quite as outrageously voluminous as FreeDB or CDDB, most of the entries that it is missing are simply bloated, redundant, or incomplete entries that MusicBrainz users have been able to avoid or quash.
The hugest asset to the MusicBrainz community's functionality is its tagger suite. Currently consisting of MusicBrainzTagger, iEatBrainz, and picard; Windows, OSX, and python based respectively, the taggers provide anyone with a huge unorganized mp3 collection an invaluable tool. The taggers use Acoustic Fingerprints, known as TRMs to locate and tag music in your collection. This automated matching system allows for gigs and gigs of music to be cataloged and tagged practically overnight.
An up and coming member of the tagger suite, picard is meant to be "the next generation" in music organization and tagging. Instead of tagging and organizing individual songs, picard sorts by album to make organizing large collections even faster and more accurate. This new tagger has been shown to have great success rates in accurately tagging mislabeled or obscure music.
MusicBrainz is a community driven project, so that means anyone willing to contribute can and should. If you start using the database and see an error, it is encouraged that you create an account and actively participate in the project. All of the artist and album information at MB is in the Public Domain so you dont have to worry about being sued. Everything else you do at MB is licensed to Creative Commons, so you don't have to worry about your hard work being jacked surreptitiously.
MusicBrainz is also part of the MetaBrainz Foundation set up in spring, 2005. The foundation was set up as a non-profit organization solely to help the progress of the MusicBrainz database.
I searched for a long time for a decent program to sort and tag my 70+ gigs of mp3s efficiently to no avail. When I found the MusicBrainzTagger, it immediately filled a void that no other library/tagger/sorter had been able to fill as it quickly identified songs correctly. MusicBrainz still has its kinks to work out, I still can't just plug my music folder into the tagger and have it spit out a list of 10,000 perfectly accurately labeled mp3s. It comes close though, and that is good enough.
check out http://www.musicbrainz.org for more info.