At one point, I was able to download and install Darwin OS from the Apple open-source website. I was able to download Darwin 8.0.1, which is the base for 10.4 (Mac OS X Tiger).
If I remember correctly, it was around the time of Leopard (10.5). I cannot find the download link now, but it ran in single user mode, and I didn't continue on to installing an actual GUI.
Found the link from here:
Darwin 8.0.1 (direct download).
And the other (older) ISOs.
You can also take a look at the PureDarwin project:
One current goal of this project is to provide a useful bootable ISO of Darwin 10.x and Darwin 9.x.
The "current" versions of Darwin are 12.4.0 (OS X Mountain Lion 10.8.4), 13.0.0 (iOS 6, and OS X Mavericks), and 14.0.0 (iOS 7), so these are definitely older versions. Darwin 10.0 is Mac OS X Snow Leopard/iOS 4, 9.0 is Mac OS X Leopard/iPhone OS 1, and 8.0 is Mac OS X Tiger/Mac OS X for Apple TV.