Historical Geography of the United States

Here is the digital version of Charles O. Paullin and John K. Wright's Atlas of the Historical Geography of the United States, first published in 1932, beautifully prepared by the Digital Scholarship Lab, University of Richmond.

There is so much good material in there, for example travel times and distances from New York City in 1800, 1830, 1857, and 1930.

Update: American Panorama is an historical atlas of the United States for the twenty-first century.

There is a similar atlas just for Massachusetts: The Historical Atlas of Massachusetts by Richard Wilkie and Jack Trager (1991), but the digital version is not quite as sophisticated.