Founding Documents of the United States

Declaration of Independence
Articles of Confederation
Constitution
Bill of Rights

Declaration of Independence

Declaration of Independence (Library of Congress)
The Declaration of Independence (National Archives and Records Administration)
The Declaration of Independence: Annotated (JStor Daily)
The Declaration of Independence (Constitutional Sources Project) -- annotated
The Declaration of Independence (The Avalon Project at the Yale Law School)

Articles of Confederation

The Articles of Confederation (Library of Congress)
The Articles of Confederation (National Archives and Records Administration)
Articles of Confederation (November 15, 1777) (Constitutional Sources Project) -- annotated
Articles of Confederation (The Avalon Project at the Yale Law School)

Constitution

United States Constitution (Library of Congress)
The Constitution of the United States (National Archives and Records Administration)
The Constitution of the United States (Constitutional Sources Project) -- annotated
Constitution of the United States (The Avalon Project at the Yale Law School)
The Founders' Constitution (University of Chicago Press)
The Records of the Federal Convention of 1787 (Farrand's Record) (Library of Congress)
The Debates in the Several State Conventions on the Adoption of the Federal Constitution (Elliot's Debates) (Library of Congress)

Bill of Rights & Other Ammendments to the Constitution

The Bill of Rights (Library of Congress)
The Bill of Rights (National Archives and Records Administration)
Bill of Rights/Amendments I-X (Constitutional Sources Project) -- annotated
The Bill of Rights (The Avalon Project at the Yale Law School)
Amendments 11-27 (National Archives and Records Administration)
Amendments XI–XXVII (March 4, 1794 to May 7, 1992) (Constitutional Sources Project) -- annotated
Amendments 11-27 (The Avalon Project at the Yale Law School)
The 14th Amendment: Annotated (JStor)