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1) 
http://www.gilderlehrman.org/teachers/ - contains primary sources, history podcasts, and easy to read overviews of periods throughout American history.
2) http://memory.loc.gov/ammem/index.html - American Memory provides free and open access written and spoken words, sound recordings, still and moving images, prints, maps, and sheet music that document the American history.
3)https://www.khanacademy.org/humanities/us-history