iPad App for Students: King of Math
King of Math is a great fast-paced game for improving skills in mental mathematics. The free version includes questions in addition and subtraction and a mixed of both. The full version ($0.99) on the other hand, includes questions on Multiplication, Division, Geometry, Fractions, Powers, Statistics and Equations.
In King of Math, the player starts as a farmer and improves as the level goes up; for example, after level 1, he becomes a merchant. The score of the player depends on how fast he can answer the questions at each level. The slower the answer, the smaller the score.
What I like about King of Math is the variety of exercises available. For instance, in subtraction, it does not just ask for the difference; it also asks for the minuend or subtrahend. This will make the player thinking under time pressure.