The National Geodetic Survey has a web service for providing geodetic reference data in shapefile format, suitable for use in GIS software. For those unfamiliar with the US NGS, they are a part of NOAA (go figure, they're not part of the USGS) that "defines and manages the national coordinate system." They're also the people who maintain the little metal "points" in the ground.
The National Spatial Reference System is a central part of the maintenance of a successful national coordinate system.
For those interested in more information on geodesy (the study of the shape of the Earth), there's a good paper entitled Geodesy for the Layman available from the NGS website.
There's also more information on the Federal Base Network available at the site.