The American Presidency Project contains government documents about the American presidents, past and present.
Encyclopaedia Iranica studies Iranian civilization in the Middle East, the Caucasus, Central Asia and the Indian Subcontinent.
The Gilder Lehrman Collection is a unique archive of primary sources in American history.
The Library of Congress has collections relating to United States history.
The Library of Congress' photostream on Flickr is a good place to find historical images. Some images are public domain and free to use. Please be sure to check the licensing terms.
The National Park Service Discover History website has information about the National Parks and American history.
Roy Rosenzweig Center for History and New Media has links to a variety of websites that have historical information.