A couple of months back, I started working on a Mind Map of Linux distributions which endeavored to show a birds eye view of all the Linux distributions and its descendents. But only when I started documenting it, did I realize the gravity of the situation. For one, there are umpteen Linux distributions and then a couple dozen more. And many Linux distributions do not have enough documentation which explains from which distribution they evolved. I found some Linux distributions which had drawn some aspects of two mainstream distributions and so on. It seemed anybody and everybody who were interested in Linux were rolling out their own distribution. But I do update the map and perhaps if time permits might complete what I started.
In the mean time, I came across this wonderful timeline of Unix OSes which also includes Linux created and maintained by Eric Levenez which is a real eye opener with respect to the sheer number of Unices out there, though he has also stated that his timeline is not complete by any respects. After all there are over 700 OSes and versions and accomodating all of them is not an easy task.
Another site which deals with the history of Unix OSes is maintained by Patrick Mulvany which also contain a nice timeline of Unix OSes in a PDF format.