National Archives and Records Administration
NARA, the National Archives and Records Administration, went online a few days ago. Their site makes available historic documents online, including the Declaration of Independence, the Constitution of the United States, and the Bill of Rights. You can read transcriptions of these documents and view high-resolution images.
The aforementioned documents are each a part of the The Charters of Freedom collection, just one of many exhibits featured on the NARA site. My personal favorite of the ones I've looked at so far is the Picturing the Century photo galleries.
They also make available a list of Milestone Documents, 100 of the most seminal documents in the history of the United States. These documents range from the Patent for Cotton Gin (1794) to the Desegregation of Central High School (1957).
Given the recent trend of vanishing government Web sites, this site is a wonderful achievement. It should be required surfing for every United States citizen. I highly recommend you check it out. If so inclined, you may want to consider a donation.