The Rare Book and Manuscript Library at Penn has quite an extensive collection of historic maps of the many neighborhoods of Philadelphia. They have been digitally captured and made available online for your viewing pleasure. The interface to view the images themselves is not that great and kind of slow, but you can generally zoom in a lot and you don't even have to go to the Library to view them. They are a rich collection of maps, survey results, redevelopment plans and other city planning data reports. Graphs, diagrams, tables and more! These are real gems and there's a romantic aire to them; they're the "vintage" of information design and visualization. The documents are sorted by location, date and title and also available via a simple map interface.