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.
Infradox serves up your archive of photo and video on a high volume basis. The Infradox API is actually two APIs: a "Partner API" and a "Back Office API." The partner API is, not surprisingly, for your partners so that they can connect live with your Infradox database. Responses are in XML.
The Stipple API is free, has no request limits as of yet, and OAUTH 2 will be supported in the future. According to its website on the API, it is "inspired by REST". Stipple announces its rare downtime, which is usually used to migrate data, via its twitter account.