• Three20 by Joe Hewitt. Hewitt wrote this library as a result of developing the Facebook iPhone application. Despite the popularity of the facebook application and the framework, he has sign said he’s done developing applications for the iPhone. This library is very extensive, but Photo Thumbnail browser and tab interface (like the Facebook application) stand out.
• Twitter Engine by Matt Gemmell. This engine will have you grabbing twitter messages and parsing twitter information in no time. The engine is built to be used for Mac and iPhone applications. Popular applications like Twitterific take advantage of the engine in their own application.
• FontLabel. The iPhone only comes equipped with a small list of fonts installed on the iPhone. With FontLabel, you can include .tff font files inside of your application resources and use the font like it where any old font in the application.
• Tapku Library. Call me bias for pointing out my own library, but I think this framework has some useful classes to help development. Update: I’ve recently added a coverflow class, along with a calendar and graph views.
• SciFiHiFi by Buzz Anderson. This framework is a small bunch of utility classes for UI and security. The UI portion has classes for loadingHUD and cells classes for showing activity and editing. The security class allows you to add usernames and passwords to the keychain.
• OpenFlow. This framework replicates the cover flow API that you see in applications like the iPod/Music app. Apple doesn’t provide a cover flow view in the SDK itself, so this framework comes in handy. (Check out my implementation of coverflow in the Tapku Library)

Frameworks are really valuable to the community because they build on an already powerful SDK. The more developers can contribute, the better applications become and the more powerful application can become.