Here is the link for an article in Genealogy Insider announcing that the Library of Congress has added 25,000 Sanborn Fire Insurance Maps to its free online collection. This is an incredible boon for historians and genealogists!

http://blog.familytreemagazine.com/insider/2017/06/01/HowToUseTheLibraryOfCongressNewSanbornMapsForGenealogy.aspx?utm_source=newsletter&utm_campaign=sft-dha-nl-170601&utm_content=947478_EDT_FTM170601&utm_medium=email

To go directly to the Sanborn Map Collection: https://www.loc.gov/collections/sanborn-maps/about-this-collection/