Our API directory now includes 203 photo APIs. The newest is the Getty Images Connect API. The most popular, in terms of mashups, is the Flickr API. We list 600 Flickr mashups. Below you'll find some more stats from the directory, including the entire list of photo APIs.
Factery is a search API with a twist. Instead of links, it returns facts. Actually, it returns links, too. But the real interesting stuff is that it extracts the sentence or two that best answers a user's search.