Open Software in a Commercial Operating System

A paper written by Wilfredo Sánchez, the then Open Source Engineering Lead at Apple, about the use of open source in commercial software such as Mac OS X Server.
http://www.mit.edu/people/wsanchez/papers/FREENIX99.html

Fink

A package manager for Darwin based on the Debian apt-get set of tools. 2900+ ports listed.
http://fink.sourceforge.net/

The Darwin Collection

A set of nearly 2000 open source packages ported to Darwin with a focus on scientific applications. A CD distribution is available.
http://www.ptf.com/tdc/

XDarwin: X Windows on Darwin and Mac OS X

A general overview of the project to bring XFree86 (X11 server) to Darwin and Mac OS X as well as pointers to download and mirror sites.
http://www.xdarwin.org/

HMUG Man Pages

An exhaustive list of Darwin man pages.
http://www.hmug.org/man/

Hexley the Platypus

The mascot of the Darwin OS.
http://www.hexley.com/

OpenDarwin

A fork of Darwin with the intention of making it easier to commit code and to involve the community in ways difficult with the main Apple branch.
http://www.opendarwin.org/

Darwin Operating System

The official Apple Darwin home page contains the current release, full source code, modification instructions, wish list, FAQ, contributors, and links.
http://developer.apple.com/darwin/projects/darwin/

The Challenges of Integrating the Unix and Mac OS Environments

A paper on the design issues Apple faced when they ported the Mac OS to a semantically different Unix base (Darwin).
http://www.mit.edu/people/wsanchez/papers/USENIX_2000/

The GNU-Darwin Distribution

A Unix distribution which focuses on the porting of free software to Darwin and Mac OS X.
http://www.gnu-darwin.org/

Getting Started With Darwin/x86

A helpful guide to getting and installing Darwin on an x86 system.
http://darwin.monkeyvoodoo.net/

Darwin Developer Documentation

Programming in the Darwin kernel and information on the differences between Darwin and other BSDs.
http://developer.apple.com/documentation/Darwin/Darwin.html

DarwinPorts

Goal is to provide an easy way to install various open-source software products on a Darwin, Mac OS X, FreeBSD, or Linux system.
http://darwinports.opendarwin.org/

MetaPkg

An alliance of Darwin packaging groups Fink, Gentoo, and DarwinPorts to facilitate delivery of freely available software to Mac OS X.
http://www.metapkg.org/