Spotify has announced that they have released the Spotify Embedded Player libspotify 9, an API to allow integration of their service into apps other than their own.
They give plenty of documentation to help developers with the integration including samples and a FAQ page.
Spotify has only been in the U.S. for a short amount of time but their innovative streaming service has been welcomed with open arms. With this new API, expect to see some of your favorite apps to have its service available within it.
Angry Birds + Spotify = amazing.