The Avalon Project is a free resource that I use on a regular basis with a couple of my US History classes. The Avalon Project, produced by Yale University, provides digital copies of hundreds of original documents from a myriad of topics in US History.

Applications for Education
The Avalon Project is a good resource for students that need to find digital copies of original documents. For example, all of The Federalist Papers are available on the Avalon Project website.