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
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.