Here is the link to the list of online music database in Wikipedia.
Also, below is a summary of music databases I have worked on. I added some notes.
MusicBrainz Database : Free and data dumps are available. (PostgreSQL), a good starting point
The Echonest API : Rosetta Stone project -> mapping multiple music APIs’ ids -> helping integrate multiple databases and APIs.
Spotify Web API : popularity data and user listening history, will integrate the Echonest API soon.
Discogs: contains label information.(http://data.discogs.com/ -> Data dumps are available.)
Quantone: (Fee) Mapping ids among spotify, discogs, and musicbrainz ids.
7Digital API: Music audio samples are available.
LastFM API: airplay data on the internet radio
Nielsen Soundscan: Not free. Traditionally used for music sales. (info from Wikipedia)
Billboard Chart Weekly Top 100: Terminate API services in 2013, chart rankings are based on Nielsen Soundcan (some scraped data from song-database.com)
Million Song database: data on music features
Musixmatch: data on lyrics
Sungyong Chang 