A total of fifteen topics are covered and include max-min problems, exponential growth, and the derivation of Kepler's second law. While this isn't exactly high school material, it's a nice site, so I'm including it anyway. This is a collection of graphical demonstrations developed for first year calculus, but suitable and interesting for high school students as well. From Douglas Arnold at Penn State. From books by Alan Selby.
Teachers can find lesson plans and classroom resources.
Tons of free math reference materials. From elementary and everyday math to algebra and beyond. Math Goodies - A site featuring interactive math lessons. It also contains chat boards that can be used to help with homework and other areas. This is a help resource for 4th through 8th grades.