Three20 is an open source project that is the brainchild of Joe Hewitt. Joe is the man behind the original Facebook application for the iPhone.
This library is an expansive collection of classes, primarily GUI related, including a flexible photoviewer (which displays local and remote images), table viewcontrollers that load content from the Internet, and URL-based navigation, among others.
Three20 includes a nice demo application that exercises many of the features of the library.
Three20 Facebook iOS Open Source
Download Three20 from github: Three20 Library
Three20 Google Group: 320
Also worth a look: Three20 Style Guide
Suggestions For Open Source Projects?
If you have written, worked with or otherwise know of an interesting open source project, one that is focused on iOS development, feel free to send me a note. Even a simple class/category that offers something useful is welcome. Contact me with ideas and suggestions.