Darwin

Darwin is the open source core of OS X. It includes the XNU kernel, along with all the free tools. XNU is the updated OpenSTEP kernel which fuses BSD 4.4 with Mach 3.0

As far as I recall it started being fully distributed in Darwin 1.4.1, which was a PowerPC only release, while later releases included support for the x86.

• Darwin 1.4.1 - OS X 10.1 - Release notes
• Darwin 6.0.1 - OS X 10.2 - Release notes
• Darwin 7.0.1 - OS X 10.3 - Release notes
• Darwin 8.0.1 - OS X 10.4 - Release notes

getting this to run

So far, under emulation I've only managed to get Darwin 8.0.1 to run under Qemu 0.13.0. As far as I know 1.4.1, has some kind of CPU issue, and 6.0.1 and 7.0.1 all have ACPI issues.