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.
Now that the Instagram API is available with last month's public beta launch, we've started to see some fun apps built on top of it. Since Instagram's only official product is an iPhone application, that means we're seeing web clients, badge widgets and even Hot-or-Not style games. The best part is that some even cross over to multiple mobile photo APIs, increasing their reach and usefulness.
The Moodstocks API provides image recognition services that developers can integrate into their mobile and web applications. The image recognition API allows developers to add reference images and define them--a process that in turn can be used when identifying objects in images.