How to Present Math: a basic how-to on presenting mathematics.
Eric Weisstein's World of Mathematics: a nice encyclopedia for math.
Planet Math: another good math encyclopedia.
Wikipedia: yet another encyclopedia for math.
The Math Genealogy Project: trace the history of you favorite mathematicians.
Robert B. Ash's online abstract algebra book. It's a pretty good book and a nice reference.
R. Lawther's Site: with another online algebra book. It's also good and a bit more elementary.
Solve E. Rubik's Cube with Abstract Algebra
Allen Hatcher's site with his famous and wonderful book on Algebraic Topology.
Bill Cowieson's site with an excellent set of online notes from a Complex Analysis class.