[nabc45]

And Doom 1/2, which has which probably is the longest list of forks and ports of any software ever: https://doomwiki.org/wiki/Source_port#List_of_source_ports

[megaman22]

Well, Doom and Quake 1 are kind of in the sweet spot where they are still small enough that one person has some hope of understanding the code base. As impressive as the later IdTech iterations are, and as awesome as it is that the code was released, they are daunting for a hobbyist.

[anc84]

Quake has more I would guess: https://en.wikipedia.org/wiki/Quake_engine#Derivative_engine... (old and incomplete)

[theandrewbailey]

In tree form: https://en.wikipedia.org/wiki/File:Quake_-_family_tree_2.svg