The Rdio iOS SDK by Rdio allows API interaction via a native playback implementation. With the SDK, developers can make calls, authenticate users, and stream audio. Available framework, documentation, and a getting started guide.
Samsung and the Tizen Association recently made the Tizen Software Development Kit for Wearables available from the Tizen developer web site. The SDK supports any wearable running Samsung's Tizen operating system, but that's an extremely limited number of devices at the moment, with just the Samsung Gear 2 and Gear 2 Neo smartwatches filling the space.