A site called SOS Math at the Univ. of
Texas at El Paso offers pages of material on topics ranging from polynomial long
division, the quadratic formula, and trigonometric identities, to Taylor polynomials,
the Cauchy-Riemann equations, and Matrix algebra.
A java-enhanced page which will allow you to zoom in on the Mandelbrot set. Seems a tad buggy, though.
Everybody seems to have a pi page. There are several pages which will allow you to search for your favorite string of numbers in some very large number of digits of pi. For example, my (home) phone number occurs beginning "at position 3619269 counting from the first digit after the decimal point".