The TwitchTV PHP SDK by Nick Law allows developers to integrate the TwitchTV API into their PHP applications. This SDK intends to support API v3, v4, and v5 until each version is deprecated. API v5 is used by default if no other is specified.
The Salsa Commons API is a REST and XML API that, as Salsa explains,
"At it's simplest, using the Salsa API may consist of just a form placed on an external website ... At it's most complex, you could use a Ruby library to authenticate, pull down information on recent supporters and events, validate that data, change it, and submit it back to the node, while generating and displaying counts of petition signers -- all through the API."
Twitter has just taken a step toward a richer social experience with its exciting new "lists" feature, which gives users the ability to organize individual accounts into groups. For example, a user can create and share a list of "celebrity" accounts that they follow. Twitter lists may be marked by their creators as public, or kept private for personal use.
For the developer seeking to experiment efficiently with social APIs, O'Reilly's 2nd Edition of "Mining the Social Web" is a truly outstanding resource.
Author Matthew A. Russell drops the developer right into the sandbox of each social network (Twitter, Facebook, LinkedIn and Google+ are particularly emphasized, as you would expect) with just the right amount of explanation about what's accessible via each dataset, and then clears out all the obstacles so they can start data mining against very clear examples.