rflickr is a Ruby implementation of the Flickr API. It includes a faithful reproduction of the published API as well as method encapsulation to provide more useful object mappings. rflickr features result caching to improve performance.
Instagram is one of those early adopter favorites that quickly crossed the chasm to more mainstream users, at least within my social network. Take that popularity, mix it with an apparent desire to buy shirts, and it was inevitable that someone would use the Instagram API to create wearable mashups. In fact, there are at least three such services, each with its own twist.
Recently launched, Everlapse, is a free iOS app for sharing images and slideshows. Communicating with images is already a winning formula in the world of social networking, but Everlapse makes it a little more interesting by providing something that is a bit more stimulating than just static photos, but not quite as complicated as videos. In a nutshell, it allows users to create and contribute to clips which are made up of various photos and displayed as a slideshow. By providing the Everlapse API, the company also makes it possible for developers to integrate this functionality with other applications.