theaudiodb.com - They require a donation of $3 each month for proper access to the API. If someone becomes a sponsor to me on GitHub I can consider it.
MusicBrainz - We already use it for automatic tag fetching, CD metadata and album covers. This is what is used when looking up CD's you load.
MusicMoz - There isn't an API as far as I can tell.
gnudb - This is a cddb service, could possibly be integrated.
opencdindex - Same as gnudb
Gracenote - This is a paid service without an open API as far as I can tell.
musopen - There isn't an API as far as I can tell. This isn't a service for metadata, but for free music.