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.


The original GURPS Illuminati was rough, but a fun book of inspiration for wild conspiratorial world-girdling secret society weirdness.
When Kenneth Hite started working for SJ Games, he produced a great collection of well-researched and well-written alternate-history weirdness:
I particularly like the “Black Schools” of GURPS Cabal.
“Like Hogwarts, but we cheerfully acknowledge that we are evil, privileged bastards!”