Even though Darwin is open-source, OS X is not, requiring you to pay money for an operating system that’s based on free components. But what if you want to use a free version of Darwin? Well, check out OpenDarwin, a free Darwin-based operating system developed by Internet Systems Consortium and Apple.
OpenDarwin, like OS X, runs on both PowerPC and x86 systems. Of course, just because OpenDarwin uses the same base operating system as OS X, it is drastically different. For one thing, it doesn’t include a desktop environment, such as GNOME or KDE, though they can be downloaded later using DarwinPorts. Also, OpenDarwin cannot run most OS X programs (including ones using Cocoa, Carbon, Quartz and Java) as many use Apple proprietary technology, though X11 programs will run in OpenDarwin.
Still, OpenDarwin does have its advantages. For one, it can run Windows programs in emulation via Darwine. Also, OpenDarwin can be used to test and develop X11 programs and experiment with the Darwin operating system without the need to purchase OS X. And you can even make OpenDarwin look like OS X by using this theme with the GNOME desktop environment. But by far the coolest thing about OpenDarwin is its mascot, Hexley.
I can live without the Cocoa stuff, if what I want is a workgroup server. But does this Open Darwin have, for instance, Bonjour? And the other stuff that lets you plunk a Mac onto a network and have it just start working?
OpenDarwin does support Bonjour, though I'm not sure if it comes built-in. You can get the source code for Bonjour and more information about it here. Also, as OpenDarwin's site says, OpenDarwin is not a full-featured operating system and their are far better operating systems for running a server (such as RedHat), though OpenDarwin does allow use of the features of the Darwin OS like no other operating system can provide.