American Library Association: Using Primary Sources on the Web
(ALA's tips on finding and evaluating primary sources)
Google's Primary Source Search
(Google recommends adding the words "digital collection" to your search)
Library of Congress: American Memory
National Archives: 100 Milestone Documents
Digital Public Library of America
The National Archives Experience: Digital Vaults
Cold War Files: Interpreting History Through Documents
Documenting the American South
Mapping the Nation (historical maps)
Archives Unbound Primary Source Databases: Use Gale database password
Archives Unbound: Civil War in Words and Deeds
Archives Unbound: National Security and the FBI Surveillance of Enemy Aliens