My favorite is Flickr API http://www.flickr.com/services/api/ http://www.flickr.com/services/api/ Provides a lot of functionality. Basically anything you can

My favorite is Flickr API http://www.flickr.com/services/api/ http://www.flickr.com/services/api/ Provides a lot of functionality. Basically anything you can do on the website you can do through the API. The names of the calls are intuitive. Provides different formats. And most importantly has libraries available for different languages that will make the calls for you.