Library of Congress: The world's largest library. Access documents, letters, journals, and images all within the public domain. 

National Archives: The United States' record keeper. All documents and materials created in the course of business conducted by the United States federal government all within the public domain. 

Teaching American History: A resource designed to support American History instruction. The site hosts a wealth of primary sources. Includes attributions and citations. 

Digital Public Library of America: This digital collection of American cultural heritage is maintained by Cleveland Public Library.