I don't remember if I passed this on before, but ACME Mapper is a good viewer
that allows you to toggle between Google aerials, USGS topos, Google street maps
and other aerial photos. For example:
http://mapper.acme.com/?ll=41.60179,-75.46572&z=15&t=S

Main site here: http://mapper.acme.com/

With the zoom, you can get close to the scale of the Penn Pilot photos and the
historic USGS mapping at http://historical.mytopo.com/

See http://historical.mytopo.com/getImage.asp?fname=hnsd92sw.jpg&state=PA
(location of earlier link at western edge of map at center.

Matt