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.
Another site covering similar material, including solved homework problems for you to practice on, is kept in Belgium.
Dan Sloughter has a web page containing Java programs for visualizing various mathematical concepts. My favorite is one which will draw the Taylor polynomial approximations for y=sin(x) .
A site called Karl's Calculus Tutor currently covers most of what would qualify as first-semester calculus, and some of the second semester, as well.
Forget a geometry formula? Check this page at Trinity College.
A Java-enabled page for generating Pascal's triangle (or rather, the last two digits of each entry, which is good enough through the 24th line). What's the pattern of the even numbers in the triangle?!