Rdio iOS SDK
From the Holy Moly department: Rdio, my favorite music streaming service, has an iOS SDK. It looks fantastic. The Rdio.framework is pretty much a plug-and-play solution. It handles all the low-level audio and network streaming stuff, and has convenient wrappers for RESTful API calls. In short, you could build a clone of the official Rdio app, and the only thing you’d have to worry about is design and caching API responses for performance.
Check out the SDK in action in Rdio’s sample iOS app on Github.
Rdio is my favorite service because it’s organized around albums. Younger folks who came of age after the age of Napster and iTunes seem to like singles or playlists. I still like to listen to whole albums, often on repeat. Rdio makes it easy to thumb through my album collection, and to snoop through the collections of people I follow.