The MUD Connector 
The MUD Connector contains an up-to-date list of over 700 MUDs. Each listing contains a description, Telnet link, code base, and home page (if one exists). Additionally, this site offers an automated submission process, a random MUD-link generator, a discussion forum, and a comprehensive search engine to help users find a particular MUD.
Educational MUD resources
This resource page contains a collection of links to information for MUD and MOO. There's an extensive education and research category as well as references to FAQs, mailing lists, publications, guides, and information on virtual worlds.
MUD Related Writings
Archived collection of MUD articles, papers, and USENET postings.
The MUD Resource Collection
Contains an index of MUD related links
The MUDdom
Created by the students at the University of Sheffield, this site provides links to documents on MUD history, how to's, types, MUDders language, characters, social aspects, and the future of these games.
Yahoo!'s MUD clients and servers page
Yahoo!'s directory of MUD clients and servers.
Yahoo!'s MUD programming page
Yahoo!'s directory of MUD programming.