The money I invested in Family Tree University's Georgia crash course has already paid for itself with this site:
Atlas of Historical County Boundaries
I have been looking for a site like this for ages! Within each state page, you can select a date and see what the county boundaries were at that time. If you have a family that seems to have moved back and forth between counties, they may not have moved at all - maybe the county lines moved instead.
It has been especially useful for locations that are unfamiliar to me, such as Indiana, Ohio, and New York state. I'm putting this site on my favorites because I will be using it frequently.