The Spotify Web Java Library provides a wrapper to the Spotify Web API. The library provides helper functions for interacting with your music library, authentication, browsing, searching, profiles, playlists, and music metadata.
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.