Coherent was a 'clean room' clone of Unix v7, originally written for the PDP-11 in 1980. It was ported to run on the IBM PC in 1983 and to the Zilog Z8000-based Commodore 900 in 1985. Coherent was somewhat famous for being audited by Bell Labs and was vetted to be free of AT&T code. Despite its low price it failed to achieve any significant market share.
A review from 1991 can be found here
Coherent has been recently opensourced, under a 3-clause BSD license.
More information about the source code to Coherent can be found at www.nesssoftware.com.
Getting it to run
Right now the easiest way is to use Qemu. I've found that a memory size of 8MB is the best, as I had several issues with 12MB. The fsck of a large disk during boot does take a long time, so you need to be patient or use a small disk.