Merc
The big clean-up of DikuMUD's source that went on to father a large share of 1990s MUDs.
DikuMUD spread fast, but its code had been written by students in a hurry and asked for more memory and patience than most people had. In 1992–93 three programmers going by Furey, Hatchet and Kahn — Michael Chastain, Mitchell Tse and Michael Quan — took it in hand. Merc tidied, rewrote and slimmed the engine until it ran on modest hardware, and it was portable enough to compile on almost anything with a C compiler.
The result was more a kit than a game, which was the point. ROM, Envy and SMAUG all built on Merc, and through them came a long line of servers that had never heard of their ancestry. The Diku licence came along for the whole ride, including its ban on charging money — which was more than many descendants bothered to honour.