This site catalogs historical USGS topo maps and makes them available as layers that you can easily toggle on and off. There is a transparency slider so you can easily compare historical maps with each other or with the current basemap. The map layers can also be downloaded.
EarthExplorer offers aerial imagery that’s unavailable elsewhere—some of it dating back to the 1950s. Fill in your search criteria and then, on the data sets tab, select “aerial photo single frames” or “high resolution orthoimagery”. The Results tab will show the available images for your search.
PASDA offers some tiles I haven’t found elsewhere, like LIDAR of the whole state, and black and white aerial imagery (à la TerraServer) from the 90s. These are great for locating old roads, trails and waterways. Note: as of July 2020, this website replaces Penn Pilot. It contains all the same imagery and more.
Hate the new Google Maps? You’re not alone. Barry Hunter has created KMLMaps (formerly ClassyGMaps), which uses the Google Maps API to restore much of the old (and extremely useful!) functionality of previous iterations of Google Maps. Use the link above in your browser or grab the source code from GitHub.