Maps created in a particular year a genealogist has interest in is worth its weight in gold. The Evans Map Room has maps drawn at various time periods in United States history.
However, we also have software which allows a user to create a historical county map of any of the contiguous states. You can pick a year and see a map of the state as it looked during that year. This is particular helpful because counties changed their boundaries and their names. You can also plot cities and towns to make their importance more relevant to readers of your genealogies.
Here are some simple maps of New England states showing extent from the late 17th century to the 20th century.
(Text and resources taken from the print edition of: Jaccaud, Robert D, comp. Passages to Family History: A Guide to Genealogical Research in the Dartmouth College Library. Rev. ed. Hanover: Dartmouth College Library, 1994).