Pennsylvania Spatial Data Access Center files on Philadelphia. Includes shapefiles from the City of Philadelphia, as well as some state resources. Important files here are Street Centerlines, local area boundaries (planning commission, police, councilmanic districts) and aerial photos.
Shapefiles and data at the census tract, county, state, and other geographic levels going back to 1790 for counties, and 1910 for tracts. Also includes some data for downloading.