The Collections Search Center is an easy search interface to the holdings of the various museums that make up the Smithsonian Institution. It links to over 7.4 million records with 568,100 images, video and sound files, electronic journals, and other resources from the Smithsonian's museums, archives, library and research holdings and collections.
Many primary sources have been digitized, and are available in full through this interface. This should make it an attractive destination for juniors working on their U.S. history thesis!
There is a helpful tutorial to get you started.
The Smithsonian Collections Search Center is also mobile-aware. Users visiting the site with their iPhone or Android devices should automatically be redirected to a mobile friendly version of the site; iPad users receive the fully formatted pages. The Smithsonian Collections Blog features interesting items, current works in progress, or “whatever catches their eye”.