MusicBrainz

MusicBrainz is a rather cool site that hosts an on-line music database. The data is intended to be "a free, on-line encyclopedia of music information". Unlike FreeDB and Gracenote, songs are identified by acoustic fingerprints. The free MusicBrainz tagger can add the ID3 data from the database into any of your MP3's and even rename the files on the fly.