Alphonse Eylenburg has taken the massive effort of creating this timeline & family tree of over 800 OSs in the history of computing.
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.
Currently, the family tree includes between 800 and 900 different operating systems.
/Via Stephen Hackett