Crackle is an Objective-C wrapper around the 37signals Campfire API. Campfire utilizes vanilla XML/JSON over HTTP and returns Account, Rooms, Search, Messages and more. Campfire is like instant messaging, but designed exclusively for groups and allows you to save transcripts, share text, files, and code in real time.
Stack Overflow, the very successful question and answer site for programmers, have now released their own API into public beta. The API actually works with all of the Stack Exchange sites, including serverfault.com and superuser.com, with more sites possibly launching in the future. The API is supported by the site http://stackapps.com.