I still maintain that one of the best APIs ever released is one of the first - flickrs[0]. Their API is easy to use, does everything you want it to and then some (did you know it provides gis polygons and shapefiles for points of geographic interest, or the work they did with machine tags).
Its sandbox and examples are best in class and honestly I've very never found a modern API that comes close to the ease of use of doing anything with the Flickr API.
[0]https://www.flickr.com/services/api/