Core Plot - graph plotting framework
Dropbox API - add Dropbox support to your apps
ShareKit adds sharing capabilities to your app. Allow users to share URLs, images, text and files with services like Facebook, Delicious, Instapaper, Twitter, and more.
OmniGroup has a set of open source frameworks for handling a wide range of tasks, including string scanning, regular expressions, data storage, preferences panels, and networking.
Three20 offers several new UI view controllers such as a photo browser, launcher, and internet-aware tables.
Nimbus offers many of the same features of Three20, with the goal of being better documented and easier to use.
Hue: A Generic Color Picker for the iPhone
Sparrow is a lightweight game development library layered over OpenGL.
cocos2d for iPhone is another 2d game framework built atop OpenGL.
Networking and Security
OAuth Consumer - an OAuth library for iOS. Useful when building client programs for APIs that require OAuth for user authentication.
ASIHTTPRequest - a CFNetwork wrapper that simplifies web communications.
AsyncSocket - when HTTP connections aren't enough, this library gives you a TCP and UDP socket library.