The amount of information here for US History classes in particular is priceless. Primary documentation is rarely so easy to find, or catalogued in a manner that is so user friendly. I see these materials as being valuable both as research sources and as springboards for discussions on a host of American History topics.
Technical Remarks:
The site is easy to navigate, but even better is the link that describes how materials are organized, for those who might be intimidated by the initial presentation.