My best friend Ananth Shrinivas is working in Sun Microsystems, Bangalore. He had produced one excellent article on the history of UNIX-based OS. Just within half-a-day, it marked a whooping 700 hits, a testimonial for its high quality. Go through it, if you have time and interest.
Personally, I found it to be biased towards his company's open Solaris OS and to some extent Linux. That's because the resources he got for this article talks more about these two OS. There are a few more flavours of UNIX-based OS, which could have been mentioned. Secondly, it takes more time to load, which is clearly not his fault. But in overall it is a good one.
P.S.: Ananth has personally explained me why the article was somewhat biased. He has also substantiated that it is one of the reasons for its success.