Trigon Dragon's Ultima Dox specialises on the documentation of those Ultimas which had relatively poor documentation, namely Ultimas 1-6 and Akalabeth. There are also other resources such as mapping fonts and disk art.
Quill Dragon's Lair specialises, as the name suggests, in written information. This includes manuals, documentation and transcripts mostly, but there are also essays, tests and a few other resources.
Gargish Dragon, as the name suggests, knows a lot about the culture of the Gargoyles and has written useful information about their language, including a Gargish dictionary. His dictionary received a link from the official Ultima Web Archive, even though that link is now old and doesn't work.
This site was made by the author of The Ultima Emulation Archive. It used to host all kind of Ultima 4 resources, including maps, walkthroughs, patches and much more.
The Notable Ultima has always been one of the top resource sites. It has unique resources such as information about the history and people of Origin, as well as complete transcripts from almost all the Ultimas. While the website has not been updated in a while (at least to my knowledge), it got its own domain in March 2004.
This site stood out from the beginning because of its awesome graphics. The site originally consisted of three parts:
Auric's Ultima Moongates was a great resource site, containing a mix of 'borrowed' resources and resources made by the webmaster himself. While the actual site is already extraordinary, the Ultima 4 Annex it hosts is probably the best Ultima 4 resource on the net, containing even a dynamic map and descriptions of important objects, among other things.
Kravlor Dragon's site was remarkable in that he hosted a transcription of the Pentology Ultima VIII Cluebook, which contained detailed maps, stats about weapons, armour and monsters, as well as a guide to magic. Although this site is now gone, an archived version may be still be reached thanks to the Internet Archive.
The Gates of the Abyss used to be a website specifically about Ultima Underworld: The Stygian Abyss.