It seems like my ancestors didn’t move around too much, but the county lines where they lived kept moving. I have an ancestor who lived in three different counties, even though he never moved! Instead, the county lines were twice redrawn. Of course, this is critical for finding records; I need to look in the records of the county where he lived at the time, not where the location is today. Dover-Foxcroft may be in Piscataquis County today, but his purchase of the family farm was recorded in Penobscot County Deeds as the land was part of Penobscot County when he purchased it.
Ed Stephan of Bellingham, Washington has created a Web site that will simplify the searches for many genealogists.
Quoting from Ed’s Web site:
When I was working on my doctorate, in the late 'sixties, I was investigating variation in the size of United States counties. At that time there was little information about the formation of U.S. counties, and I was assured by the Geography Division of the U.S. Census Bureau that very little would ever be found (e.g., there were no nationwide county maps prior to the 1930s).
Since then much work has been done in this area. My own research, which has taken me way beyond my early interest in county size, is available on this website in the form of a free, on-line book: The Division of Territory in Society.
Ed also has created a large animated GIF image that shows the formation of counties across the entire United States. It shows all county boundaries for 1650, 1700, 1750, and census years from 1790 onward. Because of its size, it can take a while to load. Once cached, however, clicking "reload" will give you quite a show.