Library of Congress - Try adding site:loc.gov to any regular Google search.
Primary Source Timeline - Click your time period to explore primary source documents
Chronicling America - Historic Newspapers from the Library of Congress
National Archives - Try adding site:archives.gov to any regular Google search.
National Park Service - Try adding site:nps.gov to any regular Google search.
Smithsonian Museum - Try adding site:si.edu to any regular Google search.
Digital Public Library of America - Great for primary sources. Try adding site:dp.la to any regular Google search.
Fordham Modern History Sourcebook - Navigate by finding your time-period or topic on the left.