HNet: Humanities and Social Sciences Online
A huge network of discussion groups that includes a job guide, course syllabi, book reviews, and extensive links to other sites.
American Historical Review
The leading historical journal in the U.S.
The History E-Book Project (ACLS)
Library of Congress: American Memory: Historical Collections for the National Digital Library
Great collections of documents, pictures, and audio recordings pertaining to American history.
Evaluating Web Sites: Criteria and Tools
A useful guide from Cornell University for separating the good stuff from the garbage on the internet.