Maps

Map Collection of the Library of Congress

http://memory.loc.gov/ammem/gmdhtml/gmdhome.html

This site contains a wide variety of maps including: Panoramic, Military Battles and Campaigns, Transportation, Cities, Counties, Surveys & Cultural Maps

Map Collections available on Ancestry.com

U.S. County Land Ownership Atlases, c. 1864-1918

Historic Land Ownership and Reference Atlases, 1507-2000

U.S. Map Collection, 1513-1990

Sample State Map Resources

Look for Plat Books and Sanborn Fire Insurance Maps in the state and county you are researching.

Missouri Plat Books 1930

Missouri Sanborn Fire Insurance Maps

Utah Sanborn Fire Insurance Maps

Wisconsin Plat Maps - South Central Counties

General United States Map Resources

U.S. County Maps 1914

Historic U.S Census Maps

Links to rotating animated maps showing all the county boundries for each census year overlayed with past and present maps so you can see the changes in county boundries.
http://www.familyhistory101.com/map_census.html

Historic U.S. County Maps
Links to rotating animated maps showing all the county boundry changes for each year overlayed with past and present maps so you can see the changes in county boundries and State Department of Transportation Maps.
http://www.familyhistory101.com/map_county.html

Google Maps

http://maps.google.com/