Darwin is the Open Source operating system from Apple that forms the basis for Mac OS X, and PureDarwin is a community project to make Darwin more usable (some people think of it as the informal successor to OpenDarwin). One current goal of this project is to provide a bootable ISO of Darwin 9.x (the version that corresponds to 10.5.x Leopard). Another goal of this project is to provide additional documentation.
The goal of this project is to make Darwin more usable by providing an installation ISO, documentation, and add-on software.
Developer Preview for the very adventurous!
Go here: http://www.puredarwin.org/