The National Archives of the United States has made available nearly 300 videos on YouTube. Their YouTube channel can be seen here: http://www.youtube.com/usnationalarchives. David S. Ferriero, the Archivist of the United States, has chosen his favorites: http://blogs.archives.gov/aotus/?p=1264.
In addition, the National Archives and Records Administration website can be found at http://www.archives.gov. You can also explore their fascinating Flickr pages at http://www.flickr.com/photos/usnationalarchives/ and their Facebook page at http://www.facebook.com/usnationalarchives.