GPO Access Core documents of U.S. Democracy
From the Articles of Confederation to the Gettysburg Address, if you're looking for an American historical document, you'll probably find it here.
American State Papers - legislative & executive Congressional documents from 1789-1838
Charters of Freedom -online exhibit of the National Archives provides links to the Bill of Rights, the Constitution of the United States and the Declaration of Independence
Constitution of the United States
The Declaration of Independence
The Federalist Papers - The Avalon Project at Yale Law School
Inaugural Addresses of the Presidents of the United States - from the Bartleby Project
The National Archives - "You don't have to go to Washington D.C. to visit the National Archives"
Our Documents A list of 100 milestone documents, compiled by the National Archives
President John F. Kennedy Assassination Records Collection National Archives collection of papers, documents, video etc. related to the assassination of John F. Kennedy