<PLEASE NOTE: Three20 has officially been shut down on May 14, 2014>
Three20 is a collection of iPhone UI classes, like a photo viewer, and general utilities, like an HTTP disk cache. Three20 is derived from the Facebook iPhone app.
Photo Viewer - TTPhotoViewController emulates Apple's Photos app with all of its flick n' pinch delight. You can supply your own "photo sources", which works similiarly to the data sources used by UITableView. Unlike Apple's Photos app, it isn't limited to photos stored locally. Your photos can be loaded from the network, and long lists of photos can be loaded incrementally.
Message composer - TTMessageController emulates the message composer in Apple's Mail app. You can customize it to send any kind of message you want. Include your own set of message fields, or use the standard "To:" and "Subject:". Recipient names can be autocompleted from a data source that you provide.
Web image views - TTImageView makes it as easy to display an image as it is in HTML. Just supply the URL of the image, and TTImageView loads it and displays it efficiently. TTImageView also works with the HTTP cache described below to avoid hitting the network when possible.
Better text fields - TTTextEditor is a UITextView which can grow in height automatically as you type. I use this for entering messages in Facebook Chat, and it behaves similarly to the editor in Apple's SMS app.
HTTP disk cache - TTURLRequest is a replacement for NSURLRequest which supports a disk cache (NSURLRequest can only cache in RAM). It has some other nice features too. HTTP posts are as easy as supplying a dictionary of parameters. The TTURL loading system can also be suspended and resumed at any time, which is a great performance helper. Network threads often fight with the UI thread, so you can suspend the network any time your app is momentarily graphically intensive.