Sanborn Maps, 1867-1970
Includes more than 660,000 large-scale of more than 12,000 American towns and cities.
David Rumsey Historical Maps
Over 11,000 historical maps primarily from North and South America, most of them adjusted for use with the GIS browser embedded within the site.
Perry-Casteneda Library Map Collection
A list of online maps from one of the best collections in the world.
Old Maps Online
Useful for doing map-based searched for digitized historical maps in other online collections.
Library of Congress Map Collections 1500-2004
"The focus of Map Collections is Americana and Cartographic Treasures of the Library of Congress; the collection is organized according to seven major categories."