I recently ran across a pretty cool map tool that helps with finding historic county boundaries in all US states.  This tool (found on Randy Major's mapping site) can be incredibly valuable to land surveyors for research purposes.  The historical county boundaries and related information used in this tool is the Atlas of Historical County Boundaries, a project of The Dr. William M. Scholl Center for American History and Culture at The Newberry Library in Chicago, Illinois. The information is included in this tool under the Creative Commons license shown on the bottom of the linked page

• Type a PRESENT-day Place and a HISTORICAL Year, then hit the "Go!" button. The place you type MUST be a PRESENT-day U.S. city, town, or county; you can even type a current day address or road name*. The year can be from the mid-1600s (depending on date of state formation) through the year 2000.
• Once you see county boundaries, click inside any county to see its name and details. The information window that appears lists the county name and when that county's then-current boundaries (as of the year you typed) went into effect. Click the < and > buttons next to the Year input box to go back and forward a decade at a time.
• Travel through time and explore the map! Click the < and >buttons next to the Year input box to go back and forward a decade at a time. Zoom in for more detail, then click the map to get information on another location, pan to another state, or type a new place. NEW: You can also check the "Show Research Locations" checkbox and see courthouses, cemeteries, churches, and libraries on the map, which can be clicked on for more information and links about that place. See this post for more information on this functionality.