The Flickr Ruby library by Maël Clérambault aims simplicity of API access. What it does is to map the methods of the official API docs. At the same time, it returns methods intuitively. This library is always updated because of the introspective features of the new methods added continuously by flickr.
Popular iPhone photo-sharing app Instagram is a prime candidate for an API, which is probably why it already has one. Though documentation refers to it as an "unofficial" Instagram API, the fact that it uses the company's own servers makes it seem pretty official. Instagram has not launched the API, nor is an API linked from its home page. Yet, it has a mobile app and right column Twitter integration, both of which likely consume the API.