A. E. Forbes Communist Collection: A site providing access to 117 documents from the papers of a rank-and-file Pittsburgh Communist, A. E. Forbes, held at the Archives Service Center at the University of Pittsburgh.
Anarchy Archives: A site devoted to the international history and theory of anarchism created by Dana Ward, Professor of Political Studies at Pitzer College (Claremont, CA).
Debs Collection and Socialist Workers Party: A site comprising approximately 2,200+ and 700+ pamphlets, respectively, held by Special Collections at Indiana State University. A limited number available for download as pdf’s.
Early American Marxism: Part of the Marxists Internet Archive that is devoted to the history of Marxist organizations in the United States from 1870s through the 1930s. The site contains extensive background information and available pdf downloads of selected documents.
Michigan State University Radicalism Collection: A large collection of material on many forms of American radicalism—right wing and left wing—with significant portions digitized.
Labadie Collection: One of the largest collections of left wing material in the US, but limited selection digitized.
This collection is hosted by the University of Pittsburgh's Digital Research Library.