Apple has a developer page that lists exactly which parts of FreeBSD they've used, and which parts of OS X are different (mainly, the parts of Mach they added t

Apple has a developer page that lists exactly which parts of FreeBSD they've used, and which parts of OS X are different (mainly, the parts of Mach they added to the FreeBSD kernel): https://developer.apple.com/library/mac/#documentation/Darwin/Conceptual/KernelProgramming/BSD/BSD.html#//apple_ref/doc/uid/TP30000905-CH214-TPXREF101 https://developer.apple.com/library/mac/#documentation/Darwi... Some of the Apple additions/inventions are also merged into FreeBSD proper: http://appleinsider.com/articles/09/10/16/freebsd_adds_support_for_snow_leopards_grand_central_dispatch.html http://appleinsider.com/articles/09/10/16/freebsd_adds_suppo...