Here are a few I like to use:
http://www.history.com/
http://americanrhetoric.com/
http://www.archives.gov/
http://www.ourdocuments.gov/
http://www.loc.gov/topics/americanhistory.php
http://americanhistory.si.edu/
http://www.usa.gov/Citizen/Topics/History-American.shtml
