National Archives of the United States. Provides digitized access to primary sources relating to all branches of the Federal Government, and more. For non-digitized resources, submit a research request.
Directory of Libraries designated as Federal Depositories. These library offer free, public access to their Federal Collections. Some provide digitized access to their collections, but other may require physical research requests.
Cornell University's Making of America Collection provides digital access to primary sources in American social history. The collection includes monographs, journals, and imprints primarily from 1840-1900.