Atlas of Historical County Boundaries
Complete data about the creation and all subsequent changes (dated to the day) in the size, shape, and location of every county in the fifty United States and the District of Columbia. It also includes non-county areas, unsuccessful authorizations for new counties, changes in county names and organization, and the temporary attachments of non-county areas and unorganized counties to fully functioning counties. Principal sources for these data are the most authoritative available: the session laws of the colonies, territories, and states that created and changed the counties.
Cartographic Boundary Shapefiles (US Census)
Pre-2010 Census Cartographic Boundary files for the Census - both political and socioeconomic boundaries available for download.