The National Archives and Records Administration maintains a great site called Charters of Freedom that maintains high-quality scans of key documents such as the Constitution and the Bill of Rights. It also includes the Declaration of Independence.
By the way, the picture in the sidebar is the National Archives building being built way-back when.
Happy Independence Day!
ADDENDUM: Wired has posted a short, interesting piece on the early mistreatment of the document, and more recent efforts to preserve it.