The United States Congressional Serial Set Map Index contains a wide variety of maps on topics as predictable as Armed Forces and Conflicts or Discovery and Exploration to those as unexpected as the Almshouses and Asylums. You can find for example the map titled "Boston in 1880, showing societies (secret and benefit), hospitals, asylums and homes." Maps included date to before 1799. For example if you were studying the Roman Empire you might find interesting a map of the "Main road system of the Roman Empire as shown by the Itinerary of Antonine (A.D. 150) and the Itinerary to Jerusalem (A.D. 333). From Parthey and Pinder."
A collection of maps which depict the commercial, industrial, and residential sections of some twelve thousand cities and towns in the United States, Canada, and Mexico and date from 1867 to the mid 1960s. The maps were used to gauge the fire risks for urban structures and display detailed information about buildings, including the outline of each building, the size, shape and construction materials, the function of structures, street names, and house and block numbers. The maps were hand-drawn and colored. These were published by the Sanborn Map Company, the primary American publisher of fire insurance maps.
This list samples historical map sites available on the web. Should you need map resources not included here please consult a reference librarian.