Hippo Campus was the number one site on The Best Social Studies Websites — 2007. It has great (and complete) online and accessible textbooks for many subjects, including History and Government. Their resources include excellent multimedia presentations.
They recently began a separate site called Hippo Campus History. It’s a more simply designed site for their U.S. History materials. I can’t tell yet if they’ve added new resources, but it’s definitely easier to navigate.