The Bandsintown Ruby SDK by Mike Constanza can access the API to implement a database of future concerts, calendars, and tickets into applications. Usage demonstrates how to find events according to artist, citiy, and venues. Documentation at http://www.bandsintown.com/api/requests
The Independent Online Distribution Alliance (IODA) has just announced its IODA Open API, currently in beta. The API offers many of the typical resources that you would expect from a music metadata service, with a few extra goodies like these album attributes: UPC code and primary label. Comparing album API results between IODA Open and iTunes, IODA is packing a lot more information into the response.
For music fans who enjoy listening to playlists on their laptops or desktops, Tomahawk is a music player with a few extra bells and whistles that may be of some interest. It's an open-source, cross-platform music player, which means it enables a user to play music stored on their own hard drives as well as from a variety of other music sources like Spotify, SoundCloud, YouTube and more. The Toma.hk API makes this functionality available to developers or users to add songs to websites.