Greater Philadelphia GeoHistory NetworkSee what your neighborhood looked like a century or two ago. This tool is "a web-based repository of geographically organized historical information about Philadelphia, its geography, its buildings, and its people."
Pro Tip: Start with the "Resource Browser" and narrow down your search by choosing categories on the left side of the brower, under either "Resource Type" or "Geography". The "Property Maps / Atlases" offers hi-res images of original atlases published from 1683-1929.