"Annenberg Classroom offers a wide array of educational resources under a single umbrella. Educators can find curricula, lesson plans, multimedia programs, and other teaching materials all indexed and in conformance with the educational standards of their state."
A commercial site, here you can find the text of many of documents regarding and surrounding the Constitution, Bill of Rights, the Declaration of Independence, and the Supreme Court. The site also sells a pocket U.S. Constitution.
"From the Law Library of Congress, links to the text of the Constitution, plus information on constitutional interpretation, executive privilege, military tribunals, national security, whistleblowers, presidential inherent powers, presidential signing statements, second amendment, state secrets privilege, war initiation, war powers, war powers resolution, and other Constitutional resources."
"The Making of America (MOA) represents an endeavor to preserve and make accessible a significant body of primary sources related to development of the U.S. infrastructure. The project began in 1995 as a collaboration between the University of Michigan and Cornell University to develop a 'thematically-related digital library documenting American social history from the antebellum period through reconstruction.' Ultimately, over 5,000 volumes with imprints between 1850 - 1877 will be included."
From the Lillian Goldman Law Library at Yale University, another offering from the Avalon Project, comes the "full text of the Federalist Papers, still a primary source for interpreting the U.S. Constitution. The 85 papers were written to advocate ratification of the Constitution and explain its meaning. Written anonymously, they are now known to have been written by Alexander Hamilton, James Madison, and John Jay."
This site from the Library of Congress' American Memory Project provides online access to documents related to the U.S. Constitution including "extracts of the journals of Congress, resolutions, proclamations, committee reports, treaties, and early printed versions of the United States Constitution and the Declaration of Independence."
From the University of Oklahoma Law Center comes this "collection of US historical documents from the Pre-Colonial period through the contemporary age, ordered by timeline. You can find the biographies of the first ladies & presidents of the United States as well as the Declaration of Independence, the inaugural addresses of the presidents, or the Emancipation Proclamation."