AMDOCS For the Study of American History

Primary Source Nexus - U.S. History Primary Sources by Time Period

Digital Public Library of America (DPLA) 

ITT Chicago-Kent College of Law OYEZ Supreme Court Database

Middle Tennessee State University - American Women Through Time

New York State Historical Newspapers

UC Santa Barbara The American Presidency Project

UH - Digital History

U of Richmond - Voting America: United States Politics, 1840-2008

Government agencies collect, preserve maintain and digitize documents relating to the culture, history and business of government of the United States.

Library of Congress - American Memory

National Archives - Docs Teach

Google Arts & Culture - American Democracy Project Particularly good for Presidential sources

HarpWeek Political Cartoons from Harper's Weekly (1857-1912)

Life Magazine Archives

PBS Freedom of US Webisodes on American History

Smithsonian - Price of Freedom Collection 

World Wide Web Virtual Library : History: United States