These collections are assembled by universities, archives, museums and other organizations for free access. You will need to give citation credit for materials used even if they are free of charge. Some collections are more US-centric than others but there are many many more sources out there.
Japanese-American Relocation Digital Archive (part of Calisphere) includes documents, photographs, oral histories and more from a variety of California educational institutions. See also the CSU Japanese American HIstory Digitization Project collection.
Women Working, 1830-1930 Harvard University's collection of books, documents and photographs recording women's employment in the United States.
Zapatistas! Documents of the New Mexican Revolution (referring to the 20th Century revolution as opposed to earlier.) Materials have been translated and some are excerpts due to difficulties with the original.
Perry-Casteñada Historical Map Collection from the University of Texas offers digital maps of the world, specific regions and a wide variety of time periods.
Library of Congress Digital Collections includes documents and images held in the Library of Congress. Options for limiting results to photographs or documents are on the left of the results list.
American Memory from the Library of Congress covering topics ranging from advertising to Westward Expansion, containing documents, photographs, music files and more.
Chronicling America contains newspapers not available in fee-based digital databases, generally smaller papers with localized coverage. Date range 1789-1963.
Calisphere is a cooperative effort between California scholarly institutions to provide access to image collections and digitized materials. A companion site is the Online Archive of California but much of the content listed in OAC is not digitized.
Internet Archive offers an interesting range of digitized materials including books, moving images, and sound files. This collection depends on volunteer contributions so the range of topics is widespread and not necessarily deep on some topics.
Avalon Project at Yale University contains primary source documents in law, history and diplomatic activities of nations around the world from ancient times to today.