Genealogists love maps! An excellent web page to locate maps is at The Newberry Library's web page. This will take you directly to the Atlas of Historical County Boundaries. The library is well known to genealogists and located in Chicago. You don't have to go to Chicago to have fun with the maps.
You can view historical state and county maps. They are interactive and by entering a date, the map will show the formation of counties within a state at that given time.
Click on Download Historical State and County Shapefiles to download maps for use with GIS programs. You can also opt to download KMZ files to use with Google Earth. These are large files and will take time to use.
While you are on their home page, be sure to look through their genealogy information in the pull down menu. Check out where your ancestor lived by checking out The Newberry Library.