A source Code Library for Mac Objective-C Applications
Creatures is a program for the exploration of evolution, natural selection, genetic programming, and artificial life for OS X 10.2 (Jaguar) or later. it's licensed under the BSD Licence.
Apple's Open Source
Apple uses a lot of Open source and funds development and changes to much of it and returns it to the open source community. Along with documentation plus some components used in Xcode and iOS. There is also a detailed list of open source components in Mac OS X.
Libdispatch - Grand Central Dispatch Open Sourced
The libdispatch project is the user space implementation of the Grand Central Dispatch API as seen in Mac OS X version 10.6 Snow Leopard. C Compiler support for Blocks is required. This is intended to be a resource for developers interested in learning more about libdispatch on Mac OS X.
MonoObjC .NET/Objective-C Bridge
This is an open source project that lets Mono running on Mac OS/X call the various Objective-C libraries such as Cocoa, WebKit through their Objective-C interface. It simplifies their use in Mono. Check the requirements closely, you'll need to have a version of Xcode installed as well as MonoDevelop. (All are free btw).
The XScreenSaver collection is provided as a DMG containing all 200+ screen saver plus sources that can be compiled with Xcode 3.1 or higher. The binaries that will run on either PPC or Intel Macs but MacOS X 10.4.0 or newer is required.