UC College Prep U.S. Government - lots of good government resources
http://www.oercommons.org/courses/u-s-government-politics/view
Holt American Government Textbook
http://go.hrw.com/hrw.nd/arbiter/pRedirect?project=hrwonline&siteId=695&pageId=6279
Textbooks
Open Stax, Rice University, free Government textbook