County lines are often a problem for genealogists researching American ancestry. Census records - one of the most valuable sources of genealogy information - are organized by counties. The problem is that county lines have changed frequently over time. More than one genealogist, including myself, has expended hours looking for records, only to discover that they were looking in the wrong county!
Several books have been published to show the movement of county boundaries. However, these books are sometimes hard to locate. Purchasing your own copy isn't cost-effective when all you want to do is look up one county. Luckily, today's Web technology and the hard work of Brian Nichols and others have provided a solution for most of the northeastern and southeastern states.
Nichols is the webmaster for both segenealogy.com (SouthEast Genealogy) and for negenealogy.com (NorthEast Genealogy). These sites have a wealth of genealogy information, but the maps are the most noticeable assets. The sites show maps of every few years in each state's history, including all the census maps through 1930. The maps apparently were created with AniMap and serve as a showcase for that product.
If you have ancestors from the southeastern U.S., look at http://www.segenealogy.com. I think you will be impressed. Likewise, those researching northeastern U.S. ancestry will want to look at http://www.negenealogy.com.