http://www.socialstudieshelp.com/ – Social Studies help for American History, Economics and AP Government. There are class notes, numerous Supreme Court case summaries and information on how to write a research paper inside.
http://www.world-newspapers.com/ – World newspapers, magazines, and news sites in English, sorted by country and region: I would not recommend this site for students searches but it is a great place for teachers to find articles for classroom use.
http://ashp.cuny.edu/ – Explores rich social and cultural history of the United States. This website includes an amazing array of art works from well-known museums, some great web projects, a media lab, and newsletter.