Darwin - the intelligent design behind OSX
Want a look under the hood of OSX? This guide to Darwin
— the open-source Unix-based kernel — will help you get your feet wet.
The guide traces Darwin's history back to the NeXT-based Rhapsody to
its current version, which is licensed under the Apple Public Source
License. Don't expect any Darwin hacks here, though there are lots of
links to other sources of info. 