TopoViewHistorical Topographic Map Collection from the USGS. Includes large and small scale maps between 1884 and 2006 in GeoPDFs and GeoTIFF, JPEG and KMZ.
American Memory Digital archive of material held at the Library of Congress.
Old Maps Online Repository of historic maps from collections around the world.
Perry-Castañeda Library Map Collection Online map collection covering the world, countries, states, cities, etc. from the University of Texas at Austin. Includes historic and modern maps. Includes color version of Sanborn Fire Insurance maps and its key to the colors and abbreviations.
Sanborn Fire Insurance Maps A Subscription database of the Sanborn Maps held in the collections of the Geography and Map Division at the Library of Congress. Over 6,000 of the sheets are available digitally. Free version, in color, at the University of Texas, Texas Sanborn maps