Maps are an integral part of any genealogist's toolbox. They can help when trying to pinpoint ancestors' locations in the midst of boundary changes. They can also bring to light factors which may have affected the lives of your ancestors.
Sanborn Fire Insurance Maps first appeared in 1867. Originally designed as a tool for insurers to assess risk and determine fire insurance premiums, they are now used by genealogists to see the towns their ancestors are from. In some cases, researchers may even be able to locate an ancestor's home, which may no longer exist in the present.
The UC Berkeley Library has put together a listing of available Sanborn and other fire insurance maps. The list is broken down by US state, with additional links for British Columbia, Quebec and Mexico. Clicking on a particular link will take you to a breakdown of cities available in that particular state or area with a note of which institution currently holds the maps.
If you go back to the front page and click on the owning institutions link, it will give you a list of the institutions, some of which are linked. I know for a fact that some of the ones that aren't linked are available for viewing online so it's worthwhile to do a quick internet search if you find one you're interested in. I've included a few below that I'm aware of.
Georgia Sanborn Maps via Digital Library of Georgia
Kentucky Sanborn Maps via Kentuckiana Digital Library
South Carolina Sanborn Maps via University of South Carolina Libraries