Mac OS X and Darwin
This information is taken from Deamonnews.org.
After leaving Apple Computer in the 1980s, Steve Jobs founded a new computer company, with a new mind set: NeXT. NeXT's operating system, NeXTStep, was an object-oriented operating system based on Carnegie-Mellon's Mach microkernel architecture, which was in turn based on the 4.2BSD release from the University of California. On top of Mach, NeXT implemented a Unix-like operating system called NeXTStep and replaced MIT's X Windows System with a new graphical interface called Display Postscript. Display Postscript allowed for very fine control of the desktop and graphical environment, however it never caught on outside of NeXT systems.
NeXT eventually quit making hardware and implemented NeXTStep for other operating systems, changing the name to OpenStep. In 1997, after nearly ten years of separation, Apple and NeXT merged giving birth to the new Apple. Apple promised the new operating system would incorporate features and design aspects from NeXTStep. Throughout MacOS versions 8 and 9, minor changes reflecting the new NeXT-based heritage started appearing on the desktop. Apple promised the real goods would lie in Mac OS X.
Mac OS X is a new operating system which has used code from many sources. The base is an updated version of Mach from CMU. On top of it is a microkernel-based BSD system similar to the one used in NeXTStep. All around, features from previous versions of MacOS have been incorporated to maintain compatibility with legacy applications. The Unix-land user utilities were derived from FreeBSD and NetBSD. Additionally, new interfaces, such as Aqua allow a new generation of programs to be created on this operating system.
Apple also gave back to the community. By open sourcing the base operating system, nicknamed Darwin, Apple has given developers a chance to learn the new system long before its scheduled release date. Apple has also allowed developers to make changes as they saw fit. Accordingly, it is possible to build and run Darwin on Intel-based computers instead of only on Apple's proprietary Macintosh hardware.
Mac OS X's greatest triumph will be in bringing BSD and Unix to the mass market. Apple computer has long held a niche among the least computer-literate in the marketplace. Presenting the user with an overwhelming and complicated system would seem like the worst possible solution. Apple's care and time spent ensuring compatibility with existing applications promises no current users will be left behind. The effort spent developing a friendly Apple interface for BSD ensures existing users will understand and follow the new system paradigm with minimal effort.