Fun 4 The Brain contains activities for Math, Science, and English. Lots to choose from here…
Academic Skills Builders is another site for quick and easy mind games.
The BBC – Schools site has a hundreds of resources across the curriculum and for all age levels. It obviously caters to the United Kingdom, but there are many lessons, videos, activities, etc. that would benefit those of us on this side of “the pond.”
Academic Earth has collected “thousands of video lectures from the world’s top scholars.” Great for high school students.