For those of you familiar with CDDB and Gracenote and the various precursors that spawned being able to identify your audio CDs by inserting them into your compuater, you may be interested in hearing about MusicBrainz.
The site/free service is a community-based open-source and open-database answer to the problem of CDDB going commercial a few years ago (thanks to Gracenote).
Most of you never see these services, because they are burried deep within programs you use, such as iTunes. However, since I spent some time in the late 1990s working on an MP3 jukebox ("daJuke"), I also spent some time looking over the contracts, etc. of Gracenote. In the end, I couldn't sign it, because it restricted the users from using ANY other form of music information database. Basically, if you wanted to add a separate network DB that contained additional information (cover art, lyrics, ratings, content warnings, etc), you might well be in violation of the license agreement if you used them in conjunction with the Gracenote service.