The Charters of Freedom. View America's founding documents at the National Archive's virtual exhibit hall. Good graphics, especially of items never exhibited in public.
http://www.archives.gov/exhibit_hall/charters_of_freedom/charters_of_freedom.html

Comparing the World's Constitutions. English text and background materials for the world's constitutions. This site contains cross-references for quick comparison of constitutional provisions.
http://www.uni-wuerzburg.de/law/index.html

Federalist Papers Search. Type in keywords to explore what the Federalist papers have to say about the issues with this dedicated search engine. This URL is case sensitive.
http://www.law.emory.edu/FEDERAL/federalist