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.
You are a major brand wanting to advertise. Do you: A) hire an advertising agency and pay big bucks for photo shoots, or B) crowdsource images from your fan base for free? The Olapic API integrates answer B into other apps.
Why did Pinterest remove its API documentation earlier this year? Well, apparently there is no rush to release an already developed API to the public yet. In fact, it may not be released for a while as Pinterest wants to avoid re-tweeting the #mistakes Twitter made in its infancy.