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.
GURPS The Prisoner. I’ve never run it, and I’m still not sure how I’d run it, but I think about it often.