Darwin

Darwin (or XNU) is the kernel used in Mac OS X. It is an open-source piece of software and is a UNIX based system. It was based on BSD.

This is the core element of the system, it does not provide the graphics system just a terminal.

Releases
There are many different places to get the releases

Official Releases
Apple uses Darwin for all of it's Mac OS X versions. It is availble from here.

Comunity Releases
Darwin is avaible from the Darwin community. These releases are unsupported by Apple and is not Mac OS X.