Another good place to check for primary sources is:
Library of Congress Digital Collections: Newspapers, photographs, documents, and other materials from the Library of Congress's extensive collections
Making of America: The Cornell University Library Making of America Collection is a digital library of primary sources in American social history from the antebellum period through reconstruction
Documenting the American South: Contains texts, images, and audio files related to southern history, literature, and culture, from the University of North Carolina libraries
The National Park Service Online Gallery: A comprehensive library of NPS images, video, audio, maps, presentations, documents and more.
Cold War International History Project: From the Wilson Center
EuroDocs: Online Sources for European History: A directory of primary sources sites compiled by Brigham Young University
World Digital Library- Provides free access to maps, photographs, manuscripts, sound recordings, and documents from around the globe
Primary Sources from DocsTeach: Thousands of online primary source documents from the National Archives to bring the past to life
The National Archives Catalog: Find online primary source material from The National Archive's Online Catalog
Our Documents: 100 Milestone Documents in American History