The U.S. Constitution 

• American Memory Documents
  Includes records of the Constitutional Convention, papers of Washington, Madison, and Jefferson, and outside links, among other resources.
• Annotated Constitution
  "This edition is a hypertext interpretation of the CRS [Congressional Research Service] text. It links to Supreme Court opinions, the U.S. Code, and the Code of Federal Regulations, as well as enhancing navigation through linked footnotes and tables of contents."
• Bill of Rights Institute
  "The mission of the Bill of Rights Institute is to educate young people about the words and ideas of America's Founders, the liberties guaranteed in our Founding documents, and how our Founding principles continue to affect and shape a free society." Lots of great resources!
• Centuries of Citizenship: A Constitution Timeline
  "An online experience highlighting some of the key dates and events that mark more than 200 years of our constitutional history. These timeline entries, taken as a whole, tell the evolving story of the U.S. Constitution and the continuing role that it plays in our lives."
• Charters of Freedom
  Text and information on the Declaration of Independence and the U.S. Constitution and Bill of Rights from the National Archives.
• Constitution of the United States
  Contains the text of proposed amendments which were never ratified as well as the complete Constitution and all amendments. (From Emory Law School.)
• The Constitution of the United States: Analysis and Interpretation
  From the Government Printing Office. Includes both HTML and pdf copies of articles containing legal analysis and opinions of the various articles and amendments of the Constitution.
• The Constitution Society
  "This site aims to eventually provide almost everything one needs to accurately decide what is and is not constitutional in most situations, and what applicable constitutions require one to do." Contains many resources and original source documents.
• Conversations on the Constitution
  "A program to stimulate conversations about the Constitution in school and community settings. Site includes conversation starters, information about how to conduct conversations, resources for further information, interactive features, and more."
• CQ Press In Context: Celebrating Constitution Day
  Free resources about the history and meaning of the U.S. Constitution, from Congressional Quarterly. Includes links.
• Exploring Constitutional Law
  "This site explores some of the great issues and controversies that surround our Nation's founding document."
• The Federalist Papers  /  The Anti-Federalist Papers
  Chronicle of the debates leading up to the formation of the present U.S. Constitution. Another copy of the Federalist Papers may be found at the U.S. Congress THOMAS site.
• First Amendment Library
  Online library "featuring comprehensive research coverage of key First Amendment issues and topics, a unique First Amendment Library and guest analyses by respected legal specialists."
• The Founders Constitution
  Online edition of a reference work about the Consitution. Includes many exerpts from letters, articles, books, and legal cases about various aspects of the Constitution and what it means.
• GPO Access: Constitution of the United States
  Contains the Constitution with analysis and commentary after each section. Written in .pdf format.
• Interactive Constitution
  Search by keyword or topic or explore selected Supreeme Court cases.
• A Multitude of Amendments, Alterations and Additions
  Online book from the National Park Service about "The Writing and Publicizing of the Declaration of Independence, the Articles of Confederation, and the Constitution of the United States".
• Wikipedia: United States Constitution
  Article about the origin and meaning of the U.S. Constitution. Includes links to the Constitution, the Bill of Rights, and other sites about the Constitution.