The National Archives has created a beautiful online exhibit called the Charters of Freedom. Listen to President Truman talk about the US Constitution or create a copy of the Declaration of Independence with your name added to the original signers in your choice of script.
The Archives is also sponsoring an interesting poll to register opinions regarding the 100 most influential documents in American history. You can vote on documents like the Federalist Papers, the Treaty of Paris, and the Gettysburg Address. Voting lasts through December 1st.
Check out biographical sketches of participants at the Constitutional Convention.
The US Dept. of Interior has also upgraded their website.