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.
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.
My mind was blown by how good the world of GURPS Reign of Steel, one of many great worldbooks from David L. Pulver.
Clearly an alternate conception of the Terminator universe, but instead of a single Skynet we get over a dozen demigod AIs who have divided the Earth between them, and each have widely varying attitudes to humanity and plans for what to do with us (all bad). This of course makes for (compared to Terminator) a richer world with more opportunities to move between different regions to keep the campaign interesting with the same characters.
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!"
@juergen_hubert GURPS I.O.U., GURPS Discworld, GURPS Faerie, GURPS Spirits, GURPS Technomancer, GURPS IST and GURPS Y2K (honourable mention for GURPS All-Star Jam)
@juergen_hubert GURPS Illuminati, GURPS Warehouse 23, GURPS Y2K, Gurps Weird War II, and GURPS IOU.
And GURPS Black Ops to link them all.
GURPS Celtic Myth, GURPS Arabian Nights, and GURPS Mars