PureDarwin
PureDarwin, now on GitHub! (Fork of https://code.google.com/p/puredarwin/)
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 useful bootable ISO of Darwin 10.x and Darwin 9.x.