Alphonse Eylenburg has created something wonderful:
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.