As a follow-up to your link to the National Museum of American History: the whole Smithsonian Institution is worth browsing through. They have lots of museums on particular historical topics, and they do an excellent job with their websites.

https://www.si.edu

If you don't mind mention of museum-like library collections, the Library of Congress has tons of historical documents/images/videos/etc.

http://www.loc.gov

And the Digital Public Library of America collects historical images, etc., from libraries all over the USA.

http://dp.la