The Avalon Project at Yale Law School: 18th Century Documents Contains several important primary-source documents, including the Articles of Confederation, Blackstone's Commentaries on the Laws of England, state constitutions, etc. Includes the Massachusetts Charter of 1691, several other charters, and the English Bill of Rights (1689), among others.

One of very few Magna Carta manuscripts to leave Britain is on display alongside the Declaration of Independence and the U.S. Constitution in the rotunda of the National Archives Building in Washington, D.C. Admission is free! But reservations are recommended. You can also get gratis (free) admission to the National Art Gallery (around the corner) and the Smithsonian (also walking distance). You should definitely go if you haven't been already.

The U.S. government has posted a very helpful scanned copy and transcript of one hundred important American documents on-line, including the U.S. Constitution and two of the most famous Federalist Papers.

The Library of Congress also has a digitized collection of Constitutional Convention of 1787 broadsides.