St. Mary's County GIS Map Server
The St. Mary's County GIS website is an excellent resource for historical research. There are multiple basemaps (current and historic) as well as photographic and satellite imagery. There are also options to overlay many types of information including property boundaries (with links to current owner and deed information), as well as locations of all historic districts, places on the National Historic Register and Maryland Historic Trust Sites. The website is located here:
Other Historic Map Resources