As the name implies, Grimpond Dragon's Ultima Cheater's Site is oriented towards cheating in Ultima games. It is not limited to that, however, as there are also other spoilers.
Arthuris Dragon's Ultima Maps and Tables incorporates some miscellaneous resources, mostly consisting of maps. The maps here are unique, and the most precious to me are the complete Ultima Underworld (1 and 2) maps and the huge computer-generated maps of Ultima 7 (parts 1 and 2).
At Arthuris Dragon's personal page are hosted the IRC logs of most of the Ultima engines.
Ultima Thule! provides lists of information about the Ultima series including price lists, ethics and characters, as well as some downloads of Ultima characters for the Infinity Engine RPGs.
Ian Albert's site has computer-generated Ultima 6 maps and Ultima 7 maps (both BG and SI). There is also an Ultima 6 Minecraft Conversion.
Read about this tool at this blog post.
