Ben Sayer tipped me on Saturday to the presence of County Formation Maps on the http://www.familyhistory101.com/ web site. These County Formation Maps use the Animap system (by permission of Goldbug, the Animap providers) and are probably the best online resource for finding how states and counties developed over the years from colonial to modern times.
Here are some screens for the County Formation Maps on the site. On the home page, there is a dropdown menu when you run your mouse over the "Maps" link (options are Census Maps, County Formation Maps and Antique Maps and Atlases):
Clicking on the "County Formation Maps" link, a list of states comes up (two screens, no overlap):
The starting point for Pennsylvania is 1673 and it shows the earliest Dutch Delaware Settlement areas. Below the map are 98 links for years that Pennsylvania county boundaries changed. Below these links are the abbreviations for all of the counties of Pennsylvania.
Here is the map for 1772. The changes from the previous map are summarized in a small text area above the map. In this case, the change from 1771 is:
This is a tremendous resource - valuable to historian and genealogist alike. The major value to the genealogist is that s/he can determine which county may hold records for a given location over time. My Carringer and Hoax families were in Westmoreland county in 1785, which was in Bedford County before 1773, and in Cumberland County before 1771, and unorganized before 1750.