The Khan Academy offers over 2400 free videos on subjects ranging from simple mathematics, to Algebra, science, and history. There are 125 practice exercises. Their mission is stated as: "to help you learn whatever you want, whenever you want, at your own pace." These videos could be used to supplement teachers' lessons, or students could be encouraged to watch them at home if they are having trouble understanding a concept.

http://www.khanacademy.org/