I animated a portion of the "Genealogical Map of the Counties" available at the Pennsylvania Historical and Museum Commission. Although it is not as useful without cities and towns, it's still fun to see how the county boundaries changed throughout history.
The counties depicted above are numbered in order, by year of formation:
Derived and adapted from the "Genealogical Map of the Counties" compiled and prepared in the land office, 1933. Tenth edition, 1985. Pennsylvania Historical and Museum Commission. Land Records at the Pennsylvania State Archives http://www.portal.state.pa.us/portal/server.pt/community/land_records/3184 downloaded June 2011.
Contact: Mike Voisin