Cohernet, a V7 Unix clone for the PC in the 1980s, was just released under a 3-clause BSD license. Link is listed below:

http://www.nesssoftware.com/home/mwc/source.php

I used it back in the day before I discovered Linux. It was quite usable and very stable. It's only fault was little networking support except UUCP.
The sources look complete and have some really neat information, especially in the documentation section.

Geoff