Easy to use CFNetwork wrapper for HTTP requests, Objective-C, Mac OS X and iPhone
Beanstalk is a simple, fast work queue.
Facebook SDK for iOS
Objective-C Twitter integration library for Mac OS X and iPhone. Official repository.
RestKit is a framework for consuming and modeling restful web resources on OS X and iPhone/iPad
socket.io v0.7.2 for ios devices
Three20 is an Objective-C library for iPhone developers
An easy way to get Twitter authenticating with OAuth on iPhone