Mac OS X
From Hackepedia
Mac OS X is a commercial UNIX like Operating System produced at Apple. Much of the kernel is based on Open Source FreeBSD. The userspace environment is very much different, and configuring certain simple things (such as NFS) can be quite a trial; other things, such as adding, say, external hard drives are quite easy. If one is interested how Mac OS X works internally it is probably wise to visit Open Darwin's site where source code is freely available as well as good documentation.
Apple Mac OS X versions and their codenames
10.0 Cheetah 10.1 Puma 10.2 Jaguar 10.3 Panther 10.4 Tiger 10.5 Leopard 10.6 Snow Leopard 10.7 Lion 10.8 Mountain Lion
