There's a lot of great educational math games out there, like
Blocksum or
DropSum, but those are puzzle games.
Jirosum, an entry into the TIGSource Adult/Educational Competition takes a different approach to the math portion and merges it with a shmup. The result is an amazingly well-designed arcade game that would not look out of place in the classroom, teaching kids math while entertaining them at the same time. After all, when educational and entertainment mix well, you get something awesome.
Jirosum follows a fox as he flies through a cave system, absorbing enemies into his floating cross. The enemies all have numbers associated with them, and when absorbed, add the number to the number currently in the cross. Shooting creates a projectile that hits all enemies in a straight line, subtracting their number from the number that was in the cross until it reaches zero, hits an enemy with a number higher than the projectile, or goes off the screen. Getting a perfect 0 (for example, collecting two ones and shooting them at a two) will net you extra points, with successive perfects giving you more.