US History http://www.digitalhistory.uh.edu/database/hyper_titles.cfm “An interactive, multimedia history of the United States from the Revolution to the present.” [As of 9/30/2010--most of the links on the digital history site are accessible for free.]
Pre-Algebra http://www.free-ed.net/free-ed/Math/PreAlg01/default.asp [Note: As of 9/30/2010 this link is a free online Pre-Algebra text.]
Note: The classzone online books are NOT free.
This is a complete online Algebra 1 textbook. You click by chapter and by subtitle.
Middle School Science Texts http://www.classzone.com/cz/find_book.htm?tmpState=&disciplineSchool=sc_ms&state=VA&x=20&y=24
Middle School Language Arts and Literature Texts http://www.classzone.com/cz/find_book.htm?tmpState=&disciplineSchool=la_ms&state=VA&x=21&y=23
Middle School History and Culture
*Note: The classzone examples are using Virginia texts, but this site also lets you search by state (as well as offering high school texts):