Wikipedia, on their page about Darwin (the operating system at the core of Mac OS X), says:

I've already tried Google and all I can find is binaries of 6.x and 7.x on VersionTracker
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I was wondering if anyone knew where Apple (on their website) is keeping the binaries.Up to Darwin 8.0, Apple released a binary installer (as an ISO image) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 computers as a standalone operating system. Minor updates were released as packages that were installed separately. Darwin is now only available as source code,[4] except for the ARM variant, which has not been released in any form separately from iPhone OS. However, the older versions of Darwin are still available in Binary form.
I've already tried Google and all I can find is binaries of 6.x and 7.x on VersionTracker