Sanborn Maps Online!
From Genealogy Gems:
Breaking news! The Library of Congress has put online nearly 25,000 additional Sanborn Fire Insurance Maps–and more are coming! Over the next three years, more will be added monthly until all 50 states are covered from the 1880s through the 1960s.
Sanborn maps show detailed information about neighborhoods, buildings, roads and more for thousands of towns in the U.S. and beyond. A sizable collection of pre-1900 Sanborn maps are already online at the Library of Congress (use the above link).