Discworld M.U.D.: This is the home page of Discworld M.U.D. You can access a lot of information about the game and play the game right from this site. There are also more links if you go to Links, then Player Links.
The Witches Guild by Rhonda: This was Clover's Discworld site, but is now run by Rhonda. It's also based around the Witches' Guild and has a lot of information.
DiscKeb: A witchy website run by Keb. Plenty of information about the Witches' Guild.
Airk's Discworld MUD page: Airk has tons of recent maps and updated maps. It's very useful for getting around.
Boot's Terrain Maps: Lost in the terrains? Trying to find new hunting grounds? Try the maps here.
The Spellcheck Project: An extensive list of wizard spells and the bonuses needed to cast them.
Sekiri's Home of Wizardry: This is a wizard only site, but it is very indepth and has a ton of information. You might even find a few ideas for Tricks from the wizard spells listed here. There is also a map for the Unseen University Library. It hasn't been updated in a while, but the information there is still useful.
Godbotherer's Site: What to know more about the Priest's guild and rituals? Check out this site.
Simidhel's Discworld Pages: The item archive on this site is excellent and so is the furniture archive. It has other useful information as well.
Discworld Pages: This is mainly a thief site, but it also has information on skills needed for learning commands, maps, valuable information about weapons and armour, and where you can find almost any item (on an NPC or in a shop).
Hezzel's Stat Site: There's a lot of useful information about stats and such as well as a stat calculator to help figure out rearranges.
Flatline's Skill Page: This site lists the skills of many players, as well as their teaching bonuses. It's great for finding the best teacher for a specific skill.
Zuroc's Discworld Site: With a currency calculator, stat calculator, street finder, and item archive, this site if great for almost any need.
Elanor's Discworld Pages: Here you can find various information about custom shops, CTF, banks, player housing, and other odd bits of info.
Discworld Monthly: A monthly newsletter about the Discworld Books by Terry Pratchett. They also have some interesting information about the books including summaries, quotes, and reviews.
The L-Space Web - A Terry Pratchett / Discworld Web Site: This site contains information about the author of the Discworld book, Terry Pratchett, plus a whole lot more.