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.
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.