Library of Congress Maps are forms part of the Sanborn Fire Insurance Map Collection in the Geography and Map Division of the Library of Congress
Google Earth: zoom in from "space" to an address, a landmark (including architectural works), or any point on the globe specified by latitude/longitude coordinates. Users can superimpose layers such as "3D buildings." Requires free download to access historical images.
Microsoft Research Maps: provides access to U.S. Geological Survey digital aerial photographs and digital topographic maps.
University of Texas at Austin Perry Castañeda Library Map Collection: index to a wide variety of online map resources including historical views of American cities.