Little more detail on Apple Computer's Darwin:
Darwin is a *nix POSIX-compatible Operating System based on FreeBSD and the Mach 3.0 Microkernal. It also shares significant codebase with NeXTStep, a product of the former NeXT.
Available as an Open Source OS under the APSL, it is available to run on both Macintosh computers and x86 systems.
Although Apple's main interest in Darwin is as the core of Mac OS X, Darwin can indeed be run as a full stand-alone OS.
As of this writing, the most up to date version is Darwin 1.4.1, which corresponds to Mac OS X 10.1
Darwin can be downloaded at opensource.apple.com