The 500px Ruby Sample Code is available to learn how to consume the API when implementing resources. It includes consumer initialization, base URL, and OAuth access token to implement photo display and storage features.
If you're looking to run a successful developer program, you should probably pay attention to Mathys van Abbe. He's the founder of Mobypicture (our Mobypicture API profile), which has just celebrated its 500th developer.
Bryan Parker founded ImageFly out of necessity: "To be honest, I was fed up with having to bounce around between image libraries when I was hammering out a project. It seems like it was GD one day, RMagick the next, and something else the next." ImageFly exists to simplify image modification. The ImageFly API grants developers many image editing tools within web properties and applications.
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.