American Memory Collection (LoC) - recordings, images, and text documents about a wide range of topics in American history, including collections that provide insight into how the dominant print culture and religious culture shaped public opinion (e.g. Sunday School Books, 1815-1865 and The 19th C. in Print: The Making of America in Books and Periodicals)
Chronicling America (NEH and LoC) - American newspapers from 1836-1922, including historical African American newspapers like The Richmond Planet. Tip: To preserve search terms for later refinement, open search results in a new tab.
David Rumsey Map Collection - high quality digitized images of more than 30,000 historical maps
Digital History Projects (U of R) - primary documents and historic data visualization (geographic focus on the south) from UR's Digital Scholarship Lab
Documenting the American South (UNC) - multifaceted archive with documents from diverse perspectives, including primary sources about Millie-Christine McCoy, conjoined twins whose lives are the subject poetry by Marilyn Nelson that will be published in the fall of 2012
Google Books Advanced Search - includes previews and full-texts of books, newspapers, and magazines. Although many full-text books are ones published prior to 1923, you'll also find full-text issues of Ebony and Jet magazines and other newer content that might surprise you. Save books to your personal Google Library by logging into your Google account.
Harvard Library's Digital Collections - includes images of factory workers in the 1930's; documents related to immigrant life from the late 1700's to the Great Depression; photos documenting the economic role of women from 1800-1930; and many collections related to other countries and cultures.
Internet History Sourcebooks (Fordham U) - collections of links to primary sources organized regionally, chronologically, and by special topics such as People with a History: An Online Guide to Lesbian, Gay, Bisexual, and Trans* History
Making of America (U of Michigan) - over 60,000 articles and documents about American social history from the antebellum period through reconstruction
NYPL Digital Gallery - over 800,000 images, maps, and more digitized by from the New York Public Library's vast collections
World Digital Library - international library collaboration - browse high quality digitized images by region, time period, or document type
American Presidency Project (UC-Santa Barbara) - over 10,000 documents; also see the Presidential Documents Guide from the National Archives or by using their Presidential Libraries search engine
Code of Virginia - search the entire Code of Virginia on the General Assembly's Legislative Information System
FBI Reading Room - declassified documents you can browse by category or search
Foreign Relations of the United States - documents from the State Department
Military Records (National Archives) - browse by conflict to learn more about online collections and search tips
National Security Archive (GWU) - explore Electronic Briefing Books on a wide array of topics that integrate analysis by their forensic archivists with images of declassified documents (links embedded in parenthetical references) attained by the NSA through their aggressive use of the Freedom of Information Act
Open CRS - reports about policy issues created for legislators by researchers at the Library of Congress
Oyez (IIC Chicago-Kent College of Law) - multimedia database of U.S. Supreme Court documents and media
African American Women Writers of the 19th Century (Schomburg Center, New York Public Library)
American Verse Project - digital texts of American poetry pre-1920
Boston's LGBTQ History - The History Project - incl. digitized editions of Gay Community News
Genealogical Research - national resources with regional emphasis on central Virginia
Lesbian Herstory Archives - audio and photo collections
Living in Limbo: Lesbian Families in the Deep South - web site for a photography exhibit that premiered at the Birmingham Civil Rights Institute
As NPR's 2012 NewsPoet project illustrated, current events can make compelling sources for documentary poetry.
In addition to sources you check regularly, consider reading news...