A wealth of links to primary source documents covering everything from Civil War Diaries to images in Britain's National Archive to medieval letters to Nazi propaganda posters. Highly recommended. (Powered by the California State University @ San Marcos)
New York Public Library Digital Gallery -
contains nearly 800,000 resources, including not only maps, letters, and photos but also prints, vintage posters, and manuscripts.
National Archives-The website of the National Archives is rich with primary source information (photos, maps, eyewitness accounts, census documents, military records, online museum exhibits, etc.) plus articles for further reading on these sources.
Library of Congress Digital Collection - "Access to print, pictorial, and audio-visual collections and other digital services" - site text
The Making of America - (U. Michigan) "MOA is a digital library of primary sources in American social history primarily from the antebellum period through reconstruction. The collection is particularly strong in the subject areas of education, psychology, American history, sociology, religion, and science and technology. The book collection currently contains approximately 8,500 books with 19th century imprints." site text
Valley of the Shadow - "In this digital archive you may explore thousands of original letters and diaries, newspapers and speeches, census and church records...Giving voice to hundreds of individual people, the Valley project tells forgotten stories during the era of the Civil War" - site text
The Digital Vault - A project of the National Archives, this interactive site allows you to browse, read, view, and connect thousands of primary source documents and images.
World Digital Library - Use an interactive map and timeline to view over 10,000 items (or search by topic, item type, or institution.)
"Search millions of photographs from the LIFE photo archive, stretching from the 1750s to today." - Site text. Use the decade icons to find photos or search for archival images with the search box.