Constitutional Convention Websites
Forms of American Justice and Government
The Avalon Project at Yale Law School has compiled tons of 18th-century documents, including the Articles of Confederation, Blackstone's Commentaries on the Laws of England, state constitutions, the Massachusetts Charter of 1691, and the English Bill of Rights (1689), among others.
Founding Documents
The U.S. government has posted a very helpful scanned copy and transcript of one hundred important American documents online, including the U.S. Constitution and two of the most famous Federalist Papers.
Convention Documents
The Library of Congress has also digitized a collection of Constitutional Convention broadsides.