This is a great site. Here's the blurb from the site:
The David Rumsey Historical Map Collection has over 15,800 maps online. The collection focuses on rare 18th and 19th century North American and South American maps and other cartographic materials. Historic maps of the World, Europe, Asia, and Africa are also represented. Collection categories include antique atlas, globe, school geography, maritime chart, state, county, city, pocket, wall, childrens, and manuscript maps. Some examples are United States map, maps New York, California map, Arizona map, America map, New York City map, Chicago map, and Colorado map. The collection can be used to study history, genealogy and family history.