The bibliography of the book Designing Virtual World's is filled with a treasure trove of [links].
The [DOME] project was an attempt at creating at a distributed object oriented execution environment, primarily for LPMud. I'm not sure of the status of this project but the site contains a description of the semi-famous [Dragon's Dinner] problem, along with a cute ascii picture. (link appears dead -- 12/31/2009)
The [Encyclopedia of MUDs: Dictionary] is intended for use by novice MUD players or as reference material for experienced users who would like to use a common terminology with the users of other MUDs. Provides information specific to MUDs, the root of command actions common to many games, slang, and technical terms. See also MudTerminology
The [Merc WebRing] Contains webpages of programmers of MERC and derivative MUDs (Envy, ROM, Smaug) that wish to share their coding efforts with the world.
The [Mud-Dev] mailing list is not platform, language or game specific, but concentrates on discussing the design and implementation of any and all MUD servers and systems. Another large related topic is game design. (link appears dead -- 12/31/2009, server exists but not this page)
The MUD Resource Collection is a list of links to many useful sources of MUD information. The links should be of interest to both "normal" users of MUDs, as well as to researchers studying MUDs and related forms of computer-mediated communications. These pages are provided as a service to the academic community.
The page can be found at http://www.godlike.com/muds/ (link appears dead -- 12/31/2009)
p2pmud is a new project designed to let people create their own MUDs or works of interactive fiction (IF) yet link them with others via a p2p network.
The page can be found at http://www.p2pmud.com (403 Forbidden message, contact site owner? -- 12/31/2009)
MUD Area building software and links.
The page can be found at http://snible.org/mud/
The WAIF programming manual. An implementation of lightweight objects for MOO.
The page can be found at http://ben.com/MOO/waif-progman.html.
Resources and forum for online role-playing.
The page can be found at http://www.webrpg.com/ (page not accessible, link dead? -- 12/31/2009)