ZZT!

This is the game that got Tim Sweeney the cash he needed to develop Jill of the Jungle and then go off of that success to bring Epic (Epic Megagames at the time) into the successful company it became. But I don't care about that because I never played Unreal or Fortnite.

ZZT came with its own editing software. Not just so you could place pieces around the board and make mazes or whatever, it contained a fairly robust scripting language you could use to make all sorts of things way beyond the scope of the original game the editor came with. Whole online communities grew up around creating and sharing these homemade games, first on BBSes and then on AoL fora and eventually on a dedicated website that's still around. Because the game/editor were distributed as shareware, there was almost no barrier to entry, and we were all just churning with ideas about how to break the engine and push the bounds of the software, of gameplay, and if narrative convention.

It was one of the most creative and community-focused times of my life, and fostered my lifelong passion for game design, something I still do as a hobby.