Docs Teach (National Archives): Thousands of primary source documents from the National Archives.
Spartacus Educational: Great resource for global history that links directly to primary source documents.
Fordham University: Full text book sources.
The Avalon Project, Documents in Law, History and Diplomacy (Lillian Goldman Law Library, Yale Law School)
LIFE Photo Archive (LIFE): Search millions of photographs from the 1750s to the present.