The InterViews library, written by John Vlissides, was for writing GUIs in C++ under X11. It came out in the early 90s. (Yes, I'm an ancient programmer)

He was one of the Gang of Four who wrote Design Patterns, but he wrote this library years before the book came out. It always seemed to me that he invented all the main patterns in the Interviews library before they were subsequently (nicely!) documented in the book (which went on to become a smash hit).

I've since realised that I much prefer functional programming (Clojure!) over OO, BUT, oh my goodness, what a truly groundbreaking and beautiful bit of work it was for its time. I read every line of code in that library, and I became a much better programmer for it.