The American Memory web site contains a group of exhibits assembled by the the Library of Congress. The Political Graveyard contains lots of information about, well, dead politicians. The Making of America web site provides scanned images of books and magazines from the 19th century. The Scout Report for Social Sciences provides links to high-quality, scholarly web sites in all of the social sciences. The History Net contains articles from several current history magazines.
History Today is a good history magazine for the non-specialist.
It's published in the United Kingdom, but its focus is not limited to British history. This Day In History, sponsored by The History Channel Today in Black History has several historical facts for each day, and a list of links to additional black history sites. Today in History, sponsored by the Library of Congress. Hyperhistory Hyperhistory has a chart format that lets you see what things were happening in a selected year or decade. It's interesting to see what things happened at the same time.