Digital Images -
http://digitalgallery.nypl.org/nypldigital/index.cfm - an excellent source for thousands of primary source images, with citations

American History

• American Civil War Collections - http://etext.virginia.edu/civilwar/ -

  Civil War letters and diary entries include transcriptions and digital images of the manuscripts.
• American Memory Timeline -

  http://lcweb2.loc.gov/ammem/ndlpedu/features/timeline/ -

  Primary sources for seven time periods of US history 1783-1968, from the Library of Congress. 
• America's Story from America's Library - http://www.americaslibrary.gov/cgi-bin/page.cgi -
  Kid-friendly version of the Library of Congress archives site.
• Digital Classroom - http://www.archives.gov/digital_classroom/  Materials from the National Archives and Records Administration (NARA).
• Documenting the American South (DAS) -http://docsouth.unc.edu/ -

  Over 1,200 primary sources documenting the cultural history of the American South from the viewpoint of Southerners.
• Our Documents - http://www.ourdocuments.gov/ - featuring milestone documents of American history from the National Archives. Includes images of original primary source documents, lesson plans, competitions, and educational resources.
• Salem Witch Trials - http://etext.lib.virginia.edu/salem/witchcraft/ - collection of primary source materials, including court records, contemporary books, maps, images, and literary works, relating to the Salem witch trials of 1692.

World History / Oral History

• EyeWitness - www.eyewitnesstohistory.com - offers historic personal narratives and other first-hand sources from Ancient history to the 20th century.
• Historical Voices - http://www.historicalvoices.org/ - database of spoken word collections spanning the 20th century.