NOAA and its Climate Analysis Branch
Pennsylvania Geologic Survey
Contains digital geospatial data, information on PA's geologic hazards and groundwater, and collection of maps and photographs.
State Data Centers
Tiger Mapping Service
US Census data available in map format
U.S. Census Bureau
US Gazetteer (US Census Bureau)
Searchable database of geographic places, with links to the Tiger Map Service.
U.S. Geological Survey (USGS)
Maps for travel or directions:
PennDOT Road Maps
County, township, borough, railroad and other maps. Detailed enough to show every street in the state. File sizes are large, but they are the most accurate maps for the state.
Displays maps of the world where statistical data is represented by the proportional size of countries.
Current Maps online:
Historical Map Collections online:
Look up names and locations of places in the US and its territories based on location or feature type.
Awesome virtual library of worldwide sites that provide maps, geographic data, and much more. Try especially their Geography Network Explorer, an interactive tool for locating geographic information.
Pennsylvania Spatial Data Access
This site contains data, maps and other geospatial information on Pennsylvania, including information on watersheds and wildlife.