The American Presidency Project is a comprehensive archive of more than 100,000 documents related to the study of the United States' Commander-in-Chief, including transcripts of debates, public papers, state of the union addresses, White House Press Briefings, party platforms and election returns, as well as audio and video recordings. [more inside]
Schaffer Library of Drug Policy - read the transcripts of hearings held on the 1937 Marihuana Tax Act, or the text of court decisions regarding drug policy, or the well-researched Consumer Unions report on licit and illicit drugs, or the differences between beer and drugs, according to Anheuser-Busch. A huge archive of materials, admittedly compiled from a pro-reform perspective.
Voices from the Days of Slavery. A collection of audio recordings made between 1932 and 1975 of African Americans known to have once been slaves. Hear Isom Moseley describe how he used to make soap, and express his opinion of the "white folks" who owned and ran the plantation where he was held. Wallace Quarterman describes his experience as a freed man in Georgia, and recounts the violent atmosphere of the Reconstruction South. Aunt Phoebe Boyd describes the demands of agricultural work. Even more narratives are available as transcripts from the companion exhibit, Born in Slavery: Slave Narratives from the Federal Writers' Project, 1936-1938 (linked to previously on Metafilter here), though some of these were unfortunately edited selectively.
The President Calling: American Radioworks (MPR) explores the secret phone tapes of Kennedy, Johnson and Nixon. AFAIK, the content is all previously available, but online, they've packaged and annotated it for ease of use. It's not exhaustive, but the moments picked out are often illuminating, showing "how each man used one-on-one politics to shape history." You might want to start here.
American Social Hygiene Posters from the University of Minnesota. Remember boys, You may think she's just your gal, but she may be everyone's pal.
In the spirit of the American Memory Digital Library or Duke's Digital Scriptorium, the American Museum of the Moving Image has a new exhibit called The Living Room Candidate- a comprehensive collection of presidential campaign ads since the 1950's.