The Flickr Ruby library by Maël Clérambault aims simplicity of API access. What it does is to map the methods of the official API docs. At the same time, it returns methods intuitively. This library is always updated because of the introspective features of the new methods added continuously by flickr.
The most popular API, in terms of mashup count, is far and away the Google Maps API, which accounts for 41% of all mashups. But when it comes to the most popular pair of APIs, Flickr and YouTube mashups are the most common. Not to be outdone, Google Maps joins Flickr in a near second place.