Official Tumblr Objective-C SDK by Tumblr. It provides interaction with the API along with the resources to work with CocoaPods and access documentation. Includes OAuth and xAuth, API client, inter-app communication, and URL schemes. By Bryan Irace.
Stannp.com is a free app that allows users to quickly and easily create their own custom-made postcards. The company provides the Stannp API that allows developers to integrate this tool into other applications. By snapping a picture or using an existing image, the user has the option to customize the image with fun effects and add the finished product to their Facebook wall, email it to a friend or have it turned into a real postcard to be posted the same day.
The upcoming Chirp conference organized by Twitter is bound to interest a new group of developers. Getting up to speed with a new API can take some research, but Twitter makes it easy with a handy list.