From the site,
MusicBrainz is a community music metadatabase that attempts to create a comprehensive music information site. You can use the MusicBrainz data either by browsing this web site, or you can access the data from a client program — for example, a CD player program can use MusicBrainz to identify CDs and provide information about the CD, about the artist or about related information. You can also use the MusicBrainz Tagger to automatically identify and clean up the metadata tags in your MP3 collections.
I’ve not gone through the whole site yet, but I did download the Tagger. To put in simple terms, the tagger scans my MP3 files, and fills out the correct metadata (Artist, Album, Track number) for me. As you know, I am an overzealous organizer, but I never did complete filling out all the details for my MP3 collection myself. Now that’s over, I can finally make full use of the Winamp Media Library.
Oh, and the whole database is open, with a client SDK for several languages provided. Could result in some cool applications.