Stereomood is a streaming music service that gives recommendations. To start, it uses a list of moods, each of which have an associated playlist. While playing songs, users can like or ban songs, much like Last.fm or Pandora. The Stereomood API allows full access to the site's functionality to developers, allowing developers to make fully usable clients on any platform.
The API is very well-designed and robust, offering access to every function call with a simple RESTful interface, using JSON-formatted responses like most APIs nowadays. The calls allow making, getting, editing, and deleting user playlists, getting song info including URLs so users can play songs, and many other things. Logins are handled using OAuth2.
The site has a bit about the history of the project:
the idea of stereomood was born in february 2008 in milan. giovanni, drained by the "apocalyptica" maurizio played in an exhaustingly loud volume, decided at any cost to provide him with an online tool to listen to, arrange and share the newest tracks from the best international music blogs.
thanks to the amazing effect of eleonora’s sweet and sour peppers – able to awaken even daniele’s brain, already 'retired' for the lethal doses of html and css bugs – a great new insight has droven us towards this first release. why don’t we arrange our mp3 files according to the people moods and activities?
This API has already led to a well-designed Android client called Eclectic. If you're interested, you can find it in the Market or download the .apk directly. I love seeing something that simply works so well being offered for free. I look forward to seeing more out of this one.