Sokikum - This multi-player social learning game is revolutionary - it engages kids with other kids in math challenges on their level and is way fun. It is also safe and the basic plan is free! Check it out - it's good for grades 1 - 6.
FunBrain Math Arcade - Your favorite game site has awesome math games. Choose a playing piece and make your way through all 25 games.
Cool Math - From pre-algebra to calculus, Cool Math lays out the rules of operation in easy to follow language. There is a lot to take in, but it can be a great resource when you are trying to remember how to graph parabolas. It even has digital scientific and graphic calculators.
Figure This! - Fun family math challenges that take the math out of the classroom and into the real world. Perfect for middle-school learners.
Math Cats - Cats are cute, but not as cute as cats that love math! The math cats lead learners through lots of interactive games and activities that focus on math concepts.