The Sanborn Fire Insurance Map Company, established in 1867, compiled and published maps of U.S. cities and towns for the fire insurance industry to assess the risk of insuring a particular property. The maps are large scale plans of a city or town drawn at a scale of 50 feet to an inch, offering detailed information on the use made of commercial and industrial buildings, their size, shape and construction material. Some residential areas are also mapped. The maps show location of water mains, fire alarms and fire hydrants. They are color-coded to identify the structure (adobe, frame, brick, stone, iron) of each building. [source]Being able to look at the buildings and area that surrounded an ancestor's homes can be very helpful. Below is an image from a St. Louis City 1909 Sanborn map showing the 1100 block of North 8th Street. My great grandfather, Herman Feinstein, was a teenager at 1122 N. 8th from 1896-1906. The 1909 map places a "Jewish School" immediately behind that address. I hypothesized that he may have gone to school there.
Where can you find Sanborn Maps? The Library of Congress has uploaded some of them [for AK, AL, AZ, DC, GA, LA, MA, MD, ME, MI, MO, NC, NE, NV, PA, TX, VA, and Mexico (Coahuila)]. And they have an index by state, county and year for their holdings in their Geography and Map Room, and they offer duplication services.
You may be able to access some through your local library. Proquest has over 600,000 of the maps, covering 1867-1970, and makes them available to libraries. (The St. Louis County Library, for example, has purchased access to the Missouri and Illinois maps, and any resident with a library card can access them from home.)
Wikipedia has a list of online archives.