This library in Java language for the Spotify Echo Nest API provides access to the Track Analysis API. With this resource, developers can obtain track's loudness, mode, key, tempo, signature, detailed beat structure, harmonic content, and timbre information. See examples with hottest songs, static playlists, and similar artists at https://github.com/echonest/jEN/tree/master/src/com/echonest/api/v4/examples
The InternetDJ API opens this electronic music site to your app. Since 1997 (not a typo!) composers and fans have been able to share and mix music and videos. According to the API's website, requests are limited to 5,000 a day, responses are in JSON and an auth key is required.
Appy Pie, cloud based mobile apps builder, has announced the inclusion of two new music APIs for its Do-it-Yourself mobile application platform. Integration with the SoundCloud and Beatport APIs will allow users to access their music libraries remotely without consuming mobile device memory.