Wolfram Demonstrations offers over 12,000 demonstrations and simulations for everything from advanced science and physics to K-2 science to arts and architecture. There is something for every teacher on this site!
School Yourself is a mathematics website based on doing math, not on watching an instructor do math. Their unique interactive lessons cover math subjects ranging from algebra, geometry, and trigonometry to precalculus and calculus. As you go through the lessons, their algorithms quietly crunch numbers and figure out where students need help the most, or whether they should breeze right through to more advanced material.
Math Planet is a resource for Div 4 teachers and their students. The educational material is focused on US math, but has support for algebra, geometry, SAT and ACT examinations. Math Planet is licensed with creative commons attribution, which means that anyone can use it.