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.

The Avalon Project, like Google Books and other resources, make it possible for me to find many primary source documents to use in my classroom. One activity that I like to do with primary documents is to distribute a collection of three to five documents about the same event or topic. Then I have students compare the viewpoints of different authors. I also have students compare the information they find in secondary sources (both online and in textbooks) with the information they see in the primary sources. For example of how one of the ways we do this, read Textbooks, Wikipedia, and Primary Source Research.

Here are some related items that may be of interest to you:
National Archives Our Documents Source Book
Documenting the American South