Kenn's Map Page is not focused towards Ultima, but also has maps of other games. As far as Ultima is concerned, it has maps of The Savage Empire (which are the best you'll ever find from that game) and Ultima 7 (parts 1 and 2).
The High Resolution Ultima Map Project, acronymed 'HRUMP' for short, has the most beautiful Ultima world maps on the net. There are maps from Ultima 1 to 8 including the Worlds of Ultima spin-offs in four different sizes.
The Lands of Ultima is a Java site showing all the maps in Ultimas 1, 3 and 5. City maps are usually shown by going to the city icon in the world map. The important places are labelled and some sort of description will appear when you move your cursor over them.
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.
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.