Development

Agora
A repository of information regarding the design and development of online multi-user environments
http://agora.cubik.org/

Art of Building
Guide, technical tips and resources for MUD building and administration.
http://aob.mudmagic.com

The Builder's Clan
Builder and administrator resource site for MUDs.
http://www.geocities.com/buildersclan/

How to Learn MOO Programming
Annotated list of tutorials in MOO programming, intended for non-programmers.
http://cmc.uib.no/moo/

MOO Database Browser
Command-line and web-based tools to view information in MOO database files which are not executing.
http://neil.fraser.name/software/moobrowser/

MOO Hardware and Software Requirements
Information about the requirements to run a MOO.
http://pythonmoo.co.uk/mirror/du.org/MOO_requirements.html

MOO Programmer's Quick Reference Sheet
A cheat sheet of the vocabulary used in MOOs. Cryptic for non-programmers.
http://cmc.uib.no/moo/docs/shorter/quick-prog-ref.txt

XYZZYnews: Dungeons, Dragons, Shovels, and Telepathy
A brief look at MUD design, by Miron Schmidt.
http://www.xyzzynews.com/xyzzy.13e.html