Darwin

See also:

Apple Developer Connection: Darwin Releases - Official page with downloads of the various Darwin releases and developer tools.
Darwin Developer Documentation - Programming in the Darwin kernel and information on the differences between Darwin and other BSDs.
Fink - A package manager for Darwin based on the Debian apt-get set of tools. 2900+ ports listed.
Getting Started With Darwin/x86 - A guide to getting and installing an old (version 6) Darwin on an x86 system.
The GNU-Darwin Distribution - A Unix distribution which focuses on the porting of free software to Darwin and Mac OS X.
Hexley the Platypus - The mascot of the Darwin OS.
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.
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). (June 19, 2000)
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. (June 10, 1999)