From the post:
I’ve recently published version 0.3.0 of my USAboundaries R package to CRAN. USAboundaries provides access to spatial data for U.S. counties, states, cities, congressional districts, and zip codes. Of course you can easily get contemporary boundaries from lots of places, but this package lets you specify dates and get the locations for historical county and state boundaries as well as city locations.
Read the full post here.