As a family historian, I'm very enthusiastic about maps. Today's Knoxville News-Sentinel has a great article about Geographic Information Systems
and their ability to communicate data effectively. [You'll probably have to register to see the article.] However, I want to mention a few of the Web sites they mentioned in the article and describe their use to genealogists. One was the Knox County (Tennessee) GIS
site. They had ward maps which would show those voting districts which roughly parallel enumeration districts on the census. Another neat site was the Geography Network
which allows users to search for an area they'd like to explore and find all sorts of maps for it. I also really liked the Appalachian Trail Conference Mapping
site which gives great explanations of the uses of GIS.