Digital History - A vast amount of material on American history, including primary sources, from the University of Houston.
U.S. Census Publications - Collections of reports obtained through the census.
Congress.gov - Includes information on bills, laws, and congressional representatives.
100 Milestone Documents - Highlights major documents in American history through 1965, from the National Archives and Records Administration.
Library of Congress - Includes plenty of material on American History in their digital collections.
Legal Information Institute: U.S. Constitution - An annotated copy of the Constitution, including relevant Supreme Court opinions, from Cornell Law School.
U.S. Presidents Study Historical Rankings - A periodic survey conducted by Siena College Research Institute ranking every U.S. president.
Check out Google's guide to refining searches to search the open web more efficiently! A few highlights: