Official Websites and Chatroom
Deryni Destination: the Official Katherine Kurtz website
The official IRC chat room:
There's a Deryni chat room where fans can gather and chat about things -- the Deryni books and other books by Katherine Kurtz, cats, food, and the like. Katherine Kurtz often makes an appearance on Sunday evenings, somewhere between 5:00 and 6:00 eastern time, so that tends to be the busiest time for the Deryni chat room.
The server is chat.psionics.net, and the channel is #Deryni_Destinations.
If you're not familiar with IRC (Internet Relay Chat) and channels and the like, start with Bynw's Quick & Dirty Guide to IRC.
Geneaologies of the XI Kingdoms
Dhassa: A fan site with fan fiction, articles and more (not recently updated)
(more to come)