David Rumsey Historic Maps - this 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.
Digital Maps Collection from the Library of Congress.
Sanborn Maps for California – A digital collection of fire insurance maps. We subscribe to the California portion, and have access to those maps which have been digitized. The Los Angeles Public Library holds a collection of the Sanborn Fire Insurance Atlases on microfilm.
The USC Libraries maintain an informative page on geographic and spatial data research.