Apple has released the source code to Darwin 9, the underlying open source operating system ofMac OS X 10.5 Leopard. “Darwin is the open source UNIX-based foundation of Mac OS X. Darwin integrates a number of technologies, including the Mach 3.0 microkernel, operating system services based on FreeBSD 5 UNIX, high-performance TCP/IP networking, and support for multiple integrated file systems. Because the design of Darwin is highly modular, you can dynamically add device drivers, networking extensions, and new file systems.”