Not every website that's valuable for MUDs falls into an easily definable category. There's histories of MUDs, descriptions of MUDs, how-to's for MUD coding, academic papers relevant to MUDs, and so on. Sites that didn't fit so easily under the other categories come here.
writing games - For a more recent take on MUDs today, this site provides a column with regular updates that offers advice on roleplaying, on player retention, on game development and more. writing games also has a webpage devoted to MUD resources that you may want to check out.
Richard A Bartle's Personal MUD Writings - Perhaps the best known of all game researchers, Richard Bartle has been studying games, especially MUDs, since the late seventies. His paper Hearts, Clubs, Diamonds, Spades is a classic of gaming studies, with its own Wikipedia page describing its influence on subsequent gaming theory.
MUD at Wikipedia - The Wikipedia contains an extensive description of the MUD genre, including a history of MUDs and their various flavors. The site also provides entries for the various major types of MUDs, for example, DikuMUD.
Online World Timeline - Raph Koster offers a timeline of online worlds as they appeared on and before the Internet, starting in 1937 with the publication of JRR Tolkien's The Hobbit. It's an interesting outline of how the worlds of fiction, gaming and computing came to intersect, culminating in Will Crowther's Colossal Cave Adventure (Advent) in 1976, the game that inspired Zork and other text-based rpgs.
Raph's site has many other good essays on text-based gaming, including his own observations of what a DikuMUD is.
MUDs at Fandom - The MUD wiki at Fandom is possibly the largest repository of articles about MUDs outside of MUD sites themselves. Currently at over 700 articles in size, the site's topics range from discussions of codebases (eg. MUSHes) to lists of resources to definitions of MUD terminology to a list of well-known mudders (ie. players of MUDs). The site also hosts descriptions of some of the more popular MUDs, including their gaming features.
TheSourcery - A site nearly as old as MUDs themselves, TheSourcery is a MUD coders' paradise, devoted to the dark arts of MUD design and development (eg. Sockets Programming). As the site owner confesses, however, much of this information may be old and outdated, especially now that newer MUDs written in Java and Python have appeared. But the curious coder may still find the site valuable.
MDA: A Formal Approach to Game Design and Research - Like Hearts, Clubs, Diamonds, Spades above, MDA is a classic paper in the gaming world, discussing the aesthetics of what makes a game 'fun'. The paper was written specifically with video games in mind, but its lessons are far-reaching, even affecting how MUDs could be written (eg. what are most MUDs but glorified obstacle courses?). If the main site is down, an archive of the paper may be found here.
The Angry GM - This and the next site are not so much devoted to MUDs as they are to roleplaying games in general, and to giving advice to game masters specifically. Which MUD administrators of all kinds can take to heart as they develop both the mechanics and the encounters in their games. One needn't be angry, for example, to learn a bit of style.
The Alexandrian - Another site devoted to dispensing advice to game masters, The Alexandrian also offers a host of other materials, including game reviews, prep notes for various Dunegons and Dragons adventures, homebrewed scenarios, comments on a currently running TTRPG (Table Top Roleplaying Game), and more. One can find generic game mastery advice on its own page, and a three clue rule method that can be useful for developing certain kinds of quests.
Grim Wheel is a site dedicated to old school text gaming.
Click the links below to go from section to section of this site.
Buildcraft - Articles which discuss the craft of building on Multi-User Dungeons (MUDs).
MUD Forums - Links to offsite forums that concern themselves with all things related to Multi-User Dungeons.
MUD-Dev - Where you can find archives of the former mailing list known as "MUD-Dev", as well as a link to the newest mailing list.
Imaginary Realities - Archives of articles from the now-defunct online magazine Imaginary Realities.
MUD Listings - Websites which list the many text games still in existence.
MUD Clients - Clients which you can use to connect to an active MUD.
Netbooks - A repository of supplements for tabletop roleplaying games.
Miscellaneous - Links that do not fall easily into any other category.
Codebases - The various public codebases in existence today.
Area Archives - Areas for public codebases such as Circle, ROM and Smaug.