I've just completed a port of UnterMUD 2.4) to various modern operating systems and compilers.
UnterMUD is a programable (U language) and distributed mud server (can network muds). It's reasonably well documented.
You can download it and more ports of ancient mud servers at MudPortsProject
You can also find these ports in the MudMagic repository.
Tyche(Comment added by Tyche on Mon Aug 15 20:38:53 2005)
I want to thank Eiz for posting a patch here that saved me a bit of time on the Linux sources. :-)