The digitized database of the Library of Congress has a diverse selection of primary source documents. Topics range from Congressional records and legislation to documentation of American folklife; the Library also provides primary sources of international interest. This is an excellent place to begin your search for primary sources.
A portrait of Louis Armstrong, taken from the Library of Congress' "Performing Arts" collection.
The National Archives has several on-line exhibits featuring primary source documents on a variety of topics pertinent to the history and culture of the U.S.. The Archives are also searchable.
The National Park Service maintains the Museum Management Program, which provides on-line access to both primary and secondary sources about a wide varity of topics related to U.S. national parks.
A key crafted from a spoon in an attempt to escape from Alcatraz. -- from the Museum Management Program
Duke University also has a number of digital collections, including collections of early advertising copy, documentary photography, and African-American history.
Yale Law School's Avalon Database consists of primary source documents which provide information about "law, history, and diplomacy;" their transcriptions span the centuries from the Code of Hammurabi to a contemporary look at these subjects.
Smithsonian Source, one of several primary source repositories maintained by the Smithsonian Museum, offers primary source documents pertaining to American history.
Image of a statue of Shiva as Lord of the Dance, as referenced in the Internet History Sourcebooks.
The Internet History Sourcebooks project, maintained by Paul Halsall at Fordham University, is a treasure trove of primary sources organized by both era and theme, as well as being searchable.
EuroDocs is a comprehensive database of newspapers, images, legislation, and other primary source documents relating to Europe. The database is searchable by country and by time period.
The New York Public Library has several on-line collections. Among them are its "Digital Gallery" and "Manuscripts and Archives" databases, both excellent sources of primary documents.
Our Documents provides access to 100 of the most influential primary source douments regarding the formation and evolution of the United States. This website is a collaborative effort by the National Archives, National History Day, and the USA Freedom Corps.
The act of Congress which established the Tennessee Valley Authority. -- from Our Documents
The Perseus Digital Library, sponsored by Tufts University, features its flagship digital collection which "covers the history, literature and culture of the Greco-Roman world." Other collections the Digital Library maintains include: Arabic language materials and "materials for the study of Germanic peoples."
Letter from Pliny the younger to Pliny the Elder. -- from the Perseus Digital Library
Making of America is a database maintained by the University of Michigan and Cornell University. The primary source documents it contains reference the United States from Antebellum times through Reconstruction.
History Matters' database, "Many Pasts," contains over 1000 primary sources documenting the lives of "ordinary Americans" beginning as early as the 1530s.
A photograph of a kindergarten class, from a book entitled Evidences of Progress Among Colored People, published in 1902 and archived in University of North Carolina's digital collection.
In addition many states, smaller universities, and museums maintain topic-specific archives of primary sources. Some examples include the Digital Library of Georgia, the Massachusetts Historical Society's "Thomas Jefferson Papers," and the University of North Carolina's digital collection which focuses on the history of North Carolina and the American South.
The University of Idaho maintains the Repositories of Primary Sources website. This is a compilation of websites, organized by country, which archive primary source material.