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.
The Rekognition API differentiates its feature set by processing images to recognize more than just the location of faces. Rekognition's technology will analyze a given photo and present a list of scenes, each with a probability rating. Faces are not simply identified, but are also classified for age and emotion.
Of all the hundreds of tags you can see in our mashup tag cloud, the second most popular behind "mapping" is "photo". How popular? As of now there are over 200 photo-related mashups listed at ProgammableWeb -- 202 by today's count.