Recently, blogging service Posterous thought it would try and help ease the burden of moving from other places on the web to its blogging platform by developing a bunch of migration tools. All of these tools were built on other services' open APIs and designed to go in, grab your content, and republish it through Posterous maintaining as much metadata as possible. Things were fine until photo-hosting service Twitpic caught wind and cut of Posterous' API access.
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.