Additional list of online primary sources:
Digital Collections – Primary documents from the Library of Congress.
Chronicling America - The pages of a range of American newspapers from 1789-1963.
The Portal to Texas History and Gateway to Oklahoma History – Archival materials on Texas and Oklahoma (including small regional newspapers) digitized by the folks at the University of North Texas.
George Washington Papers - Largest Washington collection in the world, full-text primary sources.
Making of America - Full-text primary sources from American history.
World History Matters - Scholarly articles and primary documents on world history.