FlickrNet is a .NET library for implementing all of the Flickr API's methods. It is designed to reduce the complexity of calling the API, especially with regards to authentication. This library can be accessed from a variety of frameworks and is written entirely in C#.
Kooaba offers image recognition through its Kooaba API, allowing you to compile metadata on objects in your photos. This can be used for product recognition and pricing, but could also be used to augment personal photo collections, which can be so large that they are nearly impossible to search by hand. There are tons of use cases for image recognition and this API should really open up the field and bring more applications to market.
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.