Using data from the USPS Postmaster Finder and the USGS Geographic Names Information System, geography graduate student Derek Watkins maps the opening of new post offices from 1700 to 1900. As you know, the mail must go through. No matter if it rains or snows. The mail must go through. So it's also a great way to see expansion of the US.
Some interesting spots: In 1776, after the revolution, new offices open along the east coast; in 1848, during the gold rush, offices sprout up on the west coast; in the 1870s, offices along the railroad open up.