Initially, iPhone SDK development was largely done in isolation-- the SDK doesn't support dynamic frameworks and making a static library was not obvious if you weren't already used to writing code for the Mac. As time has progressed, a number of people have published frameworks, libraries, or just plain code that does a specific task, does it well, and is intended to be integrated into other iPhone applications.

There's a number of such projects that I've found useful and, in some respects, indispensable:

PLCrashReporter ( http://code.google.com/p/plcrashreporter/ ): a library that captures application crashes so that you can then do something with them. While Apple now provides crash reports to you via iTunes connect, I use this and get immediate delivery of problems almost as they happen. In most cases, by using this, I've already been notified of a problem, fixed it, and have uploaded the corrected application before the crashes show up in iTunes connect.

JSON Framework ( http://code.google.com/p/json-framework/ ): a JSON library for Objective-C. Very useful if you're talking to a server.

ASIHTTPRequest ( http://allseeing-i.com/ASIHTTPRequest/ ): a CFNetwork based framework that makes dealing with RESTful web services easy.

Three20 ( http://github.com/joehewitt/three20/tree/master ): A framework by Joe Hewitt that provides many of the user interface components used by the current iPhone Facebook application. It's biggest shortcomings are the lack of documentation and the tight coupling between the various components.

ObjectiveResource ( http://iphoneonrails.com/ ): serialization to/from a Ruby on Rails based application using Rails standard web-services.

LLamaSettings ( http://code.google.com/p/llamasettings/ ): provides a relatively easy way of making standard looking Settings screens.

KCalendar ( http://code.google.com/p/kcalendar-iphone/ ): a simple calendar view, modeled after the built in calendar application.

What other such frameworks are you using?