GURPS was one of those RPGs I never actually played back in the day. Despite that, I owned a shelf-full of their iconic sourcebooks. I suspect many other gamers share the same story.
The GURPS sourcebooks were that useful. In a time when the internet was in its infancy, these books were a solid go-to for a quick primer on a subject, and a cornucopia of adventuring ideas. They always had a high ratio of universal RPG material to GURPS-specific stats. So it was a snap to use them with whatever game rules set.
Just got in a GURPS collection, and our shelves are loaded at the moment.
See also: Shelf photos of the week 9/2019