American Family Immigration History Center: Research facility at Ellis Island.
Documenting the American South
Populism Excellent source of documents and information on the Populist Movement of the late 19th century. (Highly recommended!)
Amdocs: Documents for the Study of American History: Documents from 15th century to Clinton's Inaugural
History and Politics Outloud: Audio recordings of important speeches and transcripts
Presidential Speeches Archives
Anti-Imperialism in the United States: 1898-1935
The Avalon Project: Documents in Law, History, and Diplomacy
Portrait of Hull House: Photographs of Jane Addams, her family, and Hull House
Gilded Age Documents: Documents from the Gilded Age of American History
American Memory: Historical Collections from the National Digital Library (this site is highly recommended)
Progressive Era Sources Online: Links to Progressive Era primary sources
AMDOCS: Documents for the study of American History
The Scopes Trial: Tennessee vs John ScopesThe famous "Monkey Trial" of 1925 which became a trial of Evolution and the right to teach it.Cold War International History Project: Excellent source of primary documents that have been compiled into bulletins. Reviews and essays on the topics also available. This page has documents from all major players in the Cold War Period
McCarthy Hearing Transcripts Recently released transcripts of the Executive Sessions of the Senate Permanent Subcommittee on Investigations of the Committee on Government Operations (McCarthy Hearings 1953-54). A must read for those who are looking for deeper insights into the instigator of the 1950's "Red Scare."