Mac OS X

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.