Constitution: Constitution Center/Interactive

http://www.constitutioncenter.org/explore/TheU.S.Constitution/index.shtml

This site translates the Constitution into Spanish and other languages, and would be useful for students to get the full impact of the document in their native language. There is also a wonderful interactive section to explore by keyword search, by topic or by Supreme Court cases. Another section gives a Constitutional timeline that includes audio clips, point/counterpoint debates, newspaper headlines and maps and graphs. This is an excellent teaching tool. I especially liked that the text is read aloud, and that it discusses the Constitution throughout the various eras. This section requires broadband.
Founding documents are also featured, including:

Magna Carta
Mayflower Compact
Virginia Declaration of Rights
Declaration of Independence
Articles of Confederation
Federalist Papers
Constitution
Bill of Rights (first 10 amendments)
Reconstruction Amendments