GURPS 3E had an amazing number of supplements on the most bizarre subjects, and I wonder what your favorite among them is.
My own nomination is for GURPS Y2K, which was published (of course) in 1999. It was basically a collection of essays on how to end the world (or at least human civilization) in style, and furthermore served as an introduction to survivalist/prepper communities.
I have a lot of love for the World of Darkness adaptations:
They not only brought a much more coherent game system (the original White Wolf system was famously bad), but also brought that GURPS worldbook rigour to the “research” that went into describing the world, the factions, the story hooks, the background, etc.
Like so many other GURPS worldbooks, I thoroughly enjoyed reading them often but never got into a game using them.