The MusicBrainz (libmusicbrainz-objc) Objective-C Library by Joachim Brandon LeBlanc is an iOS Library for the MusicBrainz Web Service. The MusicBrainz database contains a huge amount of music metadata, which is maintained by the MusicBrainz community.
ShareMyPlaylists, a Spotify playlist sharing site, has just announced the release of the ShareMyPlaylists API, allowing developers access to thousands of playlists, millions of song tracks and a wealth of playlist metadata.
In February, Morgan and Claypool Publishers released a 54-page unfinished work by Aaron Swartz on A Programmable Web. It begins with a focus on the architecture of the web, and moves on to "what it means to build a program on top of the web."