GoblinX said:
Mud Magic is one I felt would be best for this.
Mudmagic might already be a bit too heavy. It also sucks that they took the whole community down while I wasn't looking. Only their hosting service remains. Anyone knows of a reason? I really enjoyed their (non-technical) essays on mudding and RPing. Their MUDlist was great, too.
There are some other opensourced projects around, like the more simple Kmud:
http://www.kmud.de/. It seems to have not been worked on for some time, but is still quite common in use under linux mudders.
Edit: Here's another rather complex one that recently became opensource:
http://sphere.eu.org/slclient/ (German only website?)