Coolmath.com by Karen (http://www.coolmath.com/) (7th and up)
A self-proclaimed amusement park of mathematics. Those learning pre-algebra, algebra, geometry or calculus will find this site fun and helpful. Teacher links for a variety of math levels as well.

Math Advantage (http://www.harcourtschool.com/menus/math_advantage.html) (K-8th)
Choose your grade, then choose your game. This site, brought to you by Harcourt Brace, reflects certain topics from the textbook of that grade in game format.

Absurd Math Odyssey (http://www.learningwave.com/abmath) (6th and up)
Here is your chance to proceed on different missions where the ultimate power consists of mathematical skills and knowledge.

Change Maker Game (http://www.funbrain.com/cashreg/) (3rd and up)
The object is to get as much money in your piggy bank as possible--by deciding how much change you would receive from a purchase.

Brain Teasers (http://www.eduplace.com/math/brain/index.html)  (3rd/4th, 5th/6th, 7th and up)
Each week a new puzzle is presented in the different grade levels. No cheating allowed because the answer is posted a week later.

Clever Games for Clever People (http://www.cs.uidaho.edu/~casey931/conway/games.html
(1st and up)
Low-stress fun using strategy and math concepts. Definitely a cool site!

Basket Math Interactive (http://www.scienceacademy.com/BI/) (3rd and up)
From factoring, rounding, map reading and more, try this interactive basketball math.