A site that is offering a lot of great online content arranged by subject and worth checking out for the middle school to high school student is  Gooru.The library covers 5-12th grade Math and Science courses, and they are expanding to subjects like History and English Language Arts in the near future!

They sent me a message saying that they are starting with a new collection, Intro to American Government. Don't miss the interactive tour of the White House. 

Gooru has organized links that exist elsewhere, but it should really help when you are trying to organize your curriculum.