LPMUD's online editor comes with a reasonable implementation of ed. It's actually the only way to edit files in the MUD environment (and as a result, how I learned ed).
Discworld is a very customized lpmud, though there are other muds also based on Discworld's core code ("driver"). The default editor on Discworld wasn't ed, but a simpler, less-powerful line editor. ed was available as an option, though, last I checked (last time I logged in was when Pterry died, and then I hadn't logged in for years).
