How's this for dating myself: I remember playing Eamon. That's like the predecessor of MUD, and that goes back to around 1981. All MUD was, was multiplayer Eamon. Then later Ultima saw that and figured out they could make multiplayer Ultima, too, and the result was Ultima Online.
What was awesome about Eamon and MUD both was that they were completely open source, and anyone with a modicum of programming experience could dream up their own worlds. So you could pull from half your gaming community for talent to grow your universe and improve the game. No worries about graphic rendering, compatibility testing, or any of that stuff. There was no such thing as a "mod"--mods were the norm. If you didn't mod, you didn't have a game. I think I might have been 9 when I made my first dungeon. My biggest problem then was, I didn't understand play balance--the game was always rigged so I would always win.
You know what I'm disappointed about? That they never made NetHack multiplayer. The online community was totally awesome at improving on that, too. It was like mixing Linux and D&D. I think the closest they ever got to MP Nethack was Gauntlet or Legend of Zelda, but those simply didn't have the sheer wealth of "stuff" that Nethack did.