CD

The Free Track'Info'Xchange'
Free data provider for the Deluxe CD Player that comes with Windows 2000.
http://www.threetix.net/

freedb
Audio CD database using GPL software.
http://www.freedb.org/

Gracenote
The CD Data Base (formely CDDB). Enabled CD players can query for playlists.
http://www.gracenote.com/

Music Brainz
Project intended to be an OpenSource replacement for the existing system of looking up audio CDs on the Internet.
http://www.musicbrainz.org/