Alphonse Eylenburg posts an Operating Systems: Timeline and Family Tree which currently includes over 830 operating systems.
In this post you’ll find a family tree and timeline of operating systems. I have tried to include all operating systems, no matter how old or obscure. Of course, a complete list is virtually impossible, as there is no way to catalogue all the tiny hobby and embedded systems that may exist somewhere.
Please also note that I only included a few selected Linux/BSD/Solaris distributions (it is arguable whether these should count as an OS on their own or not).
Currently, the family tree includes between 800 and 900 different operating systems.
The graphic is HUGE!
And see Alphonse’s other work here.