AAAmath
Very good at quickly providing drills on major topics across the grade levels.

National Council of Teachers of Mathematics
Professional organization of mathematics teachers.  

Dr. Math
Need an explanation of a concept you never quite got,  this is a great resource.

Fibonacci Number   and Golden Ratio/Divine Proportion
An explanation of the Fibonacci sequence that plays an important role in The Da Vinci Code.  

Math World
If you love math and want to go up to university level, this is a cool website that offers all of the explanations you could possibly want in math..  

The Trends in Mathematics and Science Study
Truly a wake up call on how important it is to be spending more time helping students to develop their own understanding of math.

Women Mathematicians
Biographies of some of histories most important female mathematicians