I'm going to cheat on this one and add more detail. This is on my list because of its first story, "Catacomb," that first appeared in Dragon Magazine issue #97, published May 1985. The story made me ache with its beauty, and I wanted something like that to exist so very badly, but it was clearly just science fiction.

Imagine my amazement seven years later, walking into the UMass Darmouth Cognitive Science Lab and seeing someone playing a MUD, something I had never previously encountered. It was near enough to Catacomb as to make no difference. As it turns out, this style of software was starting to become a real thing in the 80s, but it's fair to say that most people had never encountered the concept.

en.wikipedia.org/wiki/Multi-us…