U.S. National Archives and Records Administration (NARA)
America's Historical Documents (a sampling of NARA's records, from the most celebrated milestones to little-known surprises)
Digital Library
"History Topics"
Primary Sources by State
Chronicling America: Historic American Newspapers (digitized, searchable)
Many states have statewide digital "libraries" of archival materials. Just a few examples:
Find more digital libraries with archival materials:
Digital Public Library of America
National History Day and Archives Toolkit, under Information for Students and Teachers: