Wolfram Demonstrations
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!
Wolfram Alpha Problem Generator
Practice makes perfect with the Wolfram Alpha Problem Generator. They have a huge database of problem types for you to springboard off. Additionally, their partner site, Wolfram Demonstrations offers 12,000+ interactive applications for education, research, recreation and more!
School Yourself
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.
Because of their privacy policy, students should be 18 or older in order to sign up.
Math Planet
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.