At the school I taught at last year, one of the biggest problems we faced is students becoming addicted to computer games. Although they didn't have much freedom allowed, with the internet, one website (miniclips) was allowed. If students even had 1 minute free, they would log on and it was nearly impossible to get them off once they started.
I was always on the lookout to find some sort of game that could be educational but was still fun. I'd seen ones where you answered questions and got parts to build a robot. I'd seen ones where you get a certain number right and you can color a picture. But everything I'd seen seemed pretty lame.
But I've finally found one I really liked.
I've tried several of the games there and even with a large amount of math knowledge, some of them are still difficult just due to how quickly you have to think.
My favorite is the "Save our Dumb Planet" game in which you have to match equations that link your planet to the asteroid that's going to come destroy it, and also find two points on the line. I love this because it links the skill with things it would actually be used for and does it in a sequential manner. It starts off with simple straight lines where the planet is at the origin. The it moves the planet off the origin, move the asteroid into the negative domain, gets to parabolas and even makes you distinguish which of numerous asteroidal equations is aimed at your planet!
I've made it to level 13 so far, but it's required me to bust out my calculator to check the equations.
For teachers, it looks to have statistics tools to be able to track students development so I can see this being a very fun diagnostic tool as well.