This commercial website allows free visualization to state-level data in general categories from the 1970, 1980, 1990, and 2000 census. To see more fine levels of geography or to see the 2010 census data, you have to pay for premium access.
The IPUMS consists of twenty-five high-precision samples of the American population drawn from thirteen federal censuses. This site has taken samples produced elsewhere, and has harmonized them so that users can query across samples.
This digital edition of the Foreign Relations of the United States from 1861 to 1960, with most volumes between 1900 and 1945. The digital version is not complete; there are volumes missing. This series contains the text of communications of the U.S. Department of State. It is both searchable and browsable.