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. Apple also supplies a port of X11 with Mac OS X. The current version of Darwin is 10.x, which corresponds to Mac OS X 10.6.x. All versions of Darwin are available for download.
Apple also has a short FAQ.
Darwin LinksApple Darwin Site
GNU - Darwin
Related LinksMacOS X/Darwin Binaries