This is my current favourite game right now: Brain Age. 

This is a game available for the Nintendo DS.  It is a game created by a Neurologist that follows the premise that simple, daily mental exercises can keep one's brain healthy and even prevent the early onset of neurological disease.  This premise made this game a run-away hit in Japan when it was first released in 2006, especially among older people who wanted to prevent Alzheimers disease.  I'm not sure if it can truly keep you healthy, but I do know it is a fun and addictive game. 

The game leads you through different mini-games that focus on different parts of the brain: math-fact quizzes, oral reading, drawing, memory games, and more.  This is one of only a few games I know of that is popular with both boys and girls, children and adults.  I have introduced it to both children in the school and people in their 50's, and both audiences find it enjoyable and addictive.

And anything that lets children practice their math facts can't be bad, can it?