Apple has released the Darwin source code behind its Leopard operating system to developers.

Darwin 9.0 forms the backbone of the UNIX-based operating system and is being made available to developers in the open source community, as it has been since Mac OS X 10, the first version of the OS.

Darwin 9.0 is a fully-conformant UNIX operating system that's built on Mach 3.0 and FreeBSD 5. It bundles over a hundred popular Open Source products, including bash, tcsh, ksh, and zsh, emacs, vim, and nano.

Apple continues to make all previous iterations of Darwin available, these include the Darwin used within each major version of OS X, alongside incremental upgrades.