In the most recent Weekly Genealogist (published by the New England Historic Genealogical Society), the Stories of Interest section includes a link to a CityLab article on the USGS Historical Topographic Map Explorer, which became available on Tuesday. The U.S. Geological Survey's topographic maps, which date back to 1884, are searchable.
I explored the new USGS site. You can type in the name of a location and zoom in for a close view. To view historical maps, click on a location and select a map. Once you view a map, you have the option to download it. It is possible to view areas outside of the United States as well, although you cannot overlay historical maps on them. Once you view a map, you have the option to download it. I downloaded a 1903 map of Nashville, Tennessee, a 1910 map of the New York-New Jersey Ramapo quadrangle (the area where I grew up), and an 1889 map of Chicago. I am sure I will download more maps in the future. It should be helpful to anyone wanting to locate maps of the places that their ancestors lived.