American Life Histories, 1936-1940 – Library of Congress (Federal Writers' Project) – A collection of manuscripts that describe an individual's education, income, occupation, and political views.
Clash of Cultures in the 1910s and 1920s – Ohio State University – Primary-source documents and images on prohibition, immigration, the Ku Klux Klan, the New Woman, and the Scopes Trial.
Digital Public Library - "DPLA offers a single point of access to millions of items from libraries, archives, and museums around the United States." The DPLA portal allows you to search digital collections from institutions and organizations such as Hathi Trust, Internet Archive, the Smithsonian, the National Archives, Boston Public Library and more.
HathiTrust - HathiTrust is a partnership of academic libraries and research institutions that have come together to build and share a digital repository of print works. The HathiTrust Digital Library has "more than 10 million volumes, making it one of the largest research library collections in the world. Over 3 miillion of these volumes are in the public domain and fully viewable online."
Internet Archive - The Internet Archive hosts one of the largest collections of freely available digital content on the Web and includes digitized print books, audio files, moving images and, by means of the Wayback Machine, cached copies of websites.
The New Deal Network – Franklin and Eleanor Roosevelt Institute/Columbia University – A database of over 20,000 photographs, speeches, letters, and other historic documents from the New Deal era.
Prosperity and Thrift: The Coolidge Era and the Consumer Economy, 1921-1929 – Library of Congress – A variety of sources from the 1920s that illustrate the prosperity of the Coolidge era, the nation’s transition to a mass consumer economy, and the role of government in this transition.
The Scopes Trial – University of Missouri-Kansas City Law School – An in-depth look at one of the most famous trials in American history; includes excerpts from the trial transcript as well as H. L. Mencken’s account of the trial.
Voices from the Dust Bowl - Library of Congress - A collection of audio clips, photographs, and manuscript materials that document the everyday life of central California residents in 1940-41.