All of it is nicely tied together in that, combined, the principles explain rainbows. This site provides teachers with ways to increase their effectiveness with lessons, activities, articles, and the highlighting of 13 outstanding sites every month. The Geometry Center Lots and lots of math-based computing activities. The title pretty much explains it all. You can find information by subject, region, or timeline.
Site also includes reference to related online and print resources.
Institute for Mathematics and its Applications Site dedicated to identifying problems and areas of mathematical research needed in other sciences. This is a good site, especially for those who feel they 'hate math' or can't be any good at it. It has a very good essay on the fear of math so many people seem to have and provides games and puzzles dealing with arithmetic, algebra, geometry, and probability. Another great feature is the CTK Exchange for questions and answers. The Mathematical Association of America Site of the world's largest organization devoted to the interests of collegiate mathematics.