Apple has released the core of MacOS X Server/MacOS X Client as open source. In the time since this first public release, the code base has changed dramatically. Since the graphical user interface layer is Apple proprietary code, there is a port of X11 for Darwin. All versions of Darwin are available for download.
Darwin Links
Apple Darwin SiteMacPorts
Fink
Homebrew
GNU - Darwin
Related Links
MacOS X/Darwin BinariesLinux Mac