This site covers the land and waters of the United States of America, including territories and possessions (past and present). The images are free to download, and may be used for commercial or educational purposes.
A gateway to several resources developed by the US Geological Survey: the National Atlas of the United States, US Topo, the Historical Topographic Map Collection, and the National Map Viewer.
Largest private collection of maps. Contains over 43,000 digitized maps that you can search and view through tools such as Google Earth. Cool site with focus on rare 18th and 19th century maps of North and South America.