Backpack (by 37signals) journal/status updater.

"I'll be the first to admit this code sucks. There is huge room for improvement, but this was meant to be a quick learning experience. It was about shipping an app that filled a simple need. I didn't plan on sharing it, but I've seen a lot over the last few days that has made me rethink that decision."

Tetris clone. This project is intended as an example both of general programming practices within the iPhone frameworks and of attractive interface and effective interaction design for the iPhone platform.

reMail downloads all your email to your iPhone and offers powerful full-text email search.

Even if you don't want to contribute, you may be able to use this code for your own project:

In-App Store: implements an In-App Store. I left the code in there even though it's not used any more.
Buchheit Timer: Prints timing information for blocks of code. Named for Paul Buchheit who came to YC and suggested that we time everything in our code.

github for the iphone

This is Icy, a lightweight DPKG-based installer for the iPhone. It works
on both 2.2.x and 3.x, although it may need some tuning for the latest
firmware versions.

SpeedLimit is a Leopard preference pane for limiting your network bandwidth to one of a couple different speeds — 768k DSL, Edge, 3G, and Dialup. This is really handy for testing your iPhone app under normal Edge network conditions in the iPhone Simulator.