Shades (a commercial successful MUD that ran on British Telecom from the mid 80's) happy ran with 64 users on a Z80 with 256K RAM, 2Mb RAM disk and 10Mb HDD. It

Shades (a commercial successful MUD that ran on British Telecom from the mid 80's) happy ran with 64 users on a Z80 with 256K RAM, 2Mb RAM disk and 10Mb HDD. It's actually still running (telnet://games.world.co.uk) though these days it's running on a Z80 emulator. Makes it a bit over 30 years old, and some of the old players still log in (just to see who's still around). Disclaimer: I'm the author.