Apple is taking an “open source” approach to the new “Darwin” operating system it has developed for its server computers, called Mac OS X Servers.

Darwin will function as a platform for server programs that coordinate the shared operations in work groups and other environments. It is a variant of the Unix operating system, and includes the operating system inner software “kernel” known as Mach, as well as a set of services based on the Berkeley Systems Design version of Unix.

http://www.opensource.org/