The Sanborn Maps were originally created for assessing fire insurance liability in urbanized areas in the United States. The Sanborn Company sent out legions of surveyors to record the building footprints and relevant details about these buildings in all major urbanized areas regarding their fire liability. The maps include detailed information regarding town and building information in approximately 12,000 U.S. towns and cities from 1867 to 1970. The detail is amazing; every building is shown and, in most cases, the name of the building's owner is also shown. You may find your ancestor's house and perhaps his or her place of business as well.
The maps are a highly useful resource for historical research, planning, preservation, genealogical research, sociological studies and research of urban growth.
At this time, the first volumes of the Insurance Maps of Cincinnati, Ohio are currently online and more will be made available as the collection is digitized.
You can view them at the following url: http://virtuallibrary.cincinnatilibrary.org/VirtualLibrary/vl_Maps.aspx.
My thanks to Daniel Appel for telling me about this resource.
Some other online sources for Sanborn Fire Maps include:
Digital Sanborn Maps, 1867-1970--California cities UCB only Digital maps created from ProQuest Information and Learning's microfilm collection of originals held by the Library of Congress.
Sanborn Fire Insurance Company Maps of Florida A collection of more than 3,000 map sheets digitized from the printed maps held in the collections of the Smathers Libraries' Map & Imagery Library, at the University of Florida.
Sanborn Fire Insurance Maps for Georgia Towns and Cities, 1884-1922 A digital collection of 4,445 Sanborn fire insurance maps from the University of Georgia Libraries.
Digital Sanborn Fire Insurance Maps of Utah, from the collection of the University of Utah's J. Willard Marriott Library.