Desmos - An online graphing calculator that is very user friendly and powerful
Cut-the-Knot.com - Interactive Math Puzzles and Entertaining Math Essays
Everett Kaser - Excellent Logic Puzzles
Pirates Net - Find classes, see your grades, add or drop
Graph Paper! - That's right, a web page where you can print out graph paper
Steven Dutch - A "rant" that I feel is right on target
Powers of Ten - This old video from the '70s will blow your mind, and help you understand the scale of the universe
Cosmic Eye - A recent version of Powers of Ten, with more modern science and higher quality video
Pendulum Wave (scroll down) - This video shows a set of pendulums of different lengths. It illustrates the idea of common multiples ... and it's pretty!
Khan Academy - LOTS of practice in all sorts of different mathematical subjects. It works like a game, where you build a progress bar and "level up"!
Simplex Method - Here is an online calculator that will perform the Simplex Algorithm (from Finite Math - Math 130). It shows each tableau at each step of solving, which most online calculators don't include. Thanks go to the authors Waner and Costenoble for allowing this tool from their textbook's online site to be publicly accessed.