Flickcurl is a library for implementing the Flickr API in C. This library supports the entire Flickr API, including functions for browsing, searching, and uploading photos and videos. It comes with a program called flickrdf that turns a photo's metadata, tags, machine tags, and places into RDF triples descriptions.
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.
Animetrics, a leading developer of 2D to 3D face recognition and face creation solutions, provides the Animetrics Face Recognition that can be used to detect human faces in pictures and match them against a set of known or documented faces. This tool is specifically useful for applications for the fashion and online entertainment industries, or for law enforcement or military intelligence where accurate identity resolution is a necessity.