H-NET (Humanities and Social Sciences Online) has articles, bibliographies, discussions, and more on a number of topics. It is easy to navigate to materials on a specific area of interest.
Ehistory, hosted by the Ohio State University, is a collection of original articles and primary sources covering most time periods and places.
Contingent is a new online history magazine whose contributors are primarily not full-time tenured faculty.
British Pathe videos includes newsreels produced from about 1918 to 1970. Many newsreels -- including those produced by other agencies -- are also available on YouTube. (Add the term "newsreel" to your topic in the YouTube search box: for instance, "Stalin newsreel".) [Newsreels were short documentary films of current events, usually shown in movie theatres before the feature film. Television news made them obsolete by the late 1960s, but they provide an interesting, although often biased, look at the first half of the twentieth century.]
HistoryNet: the website of the popular magazine publisher.
Alpha History: An online textbook covering eleven major events (e.g., World War I, Northern Ireland and the Troubles). Includes timelines, primary source documents, lists of major figures, and other features. It's a good choice for brushing up before moving to more in-depth study.
Rulers: A reference on heads of state.
World History Encyclopedia -- formerly the Ancient History Encyclopedia
The History Chicks: Essays and podcasts on women's history.
Zinn Education Project: Although primarily aimed at teachers, this site (based on Howard Zinn's People's History of the United States and other works) is useful for beginning historians as well.
American Archive of Public Broadcasting: a collaboration between the Library of Congress and WGBH: A collection of audio and video programs from public radio and television. As of early 2020, approximately 52,000 programs are available online; brief descriptions of other programs are available. Political and social issues are covered in greater depth than other areas.
Early Americas Digital Archive: Transcriptions of material written in or about the Americas from 1492 to about 1820. Includes letters and works from such notables as Christopher Columbus and Benjamin Franklin.
Documenting the American South (University of North Carolina): "Primary resources for the study of southern history, literature, and culture."
Civil Rights Digital Library: video clips and photographs from the 1950s and 1960s, articles, and lesson plans about the civil rights movement.
New-York Historical Society, Civil War Treasures
O Say Can You See? Stories from the National Museum of American History (blog)
100 Milestone Documents: Digital images of 100 important documents in US history, ranging from the Declaration of Independence to the Voting Rights Act of 1965.
CIA publications post-World War II monographs on U.S. foreign relations.
Library of Congress digital collections: a vast collection of primary sources, organized by topic and format.
Presidential Libraries
The American Presidency Project: documents (including party platforms and speeches), statistics, and analysis of presidential activity.
Public Papers of the Presidents: pdf files published by the National Archives, from Hoover through Obama (with the exception of FDR). Includes the presidents' journals and calendars, messages to Congress, radio addresses, and more.
The Miller Center at the University of Virginia has biographies and primary source documents on American presidents.