Alien Equation is a game that is great for elementary students who use iPod touch. While there are plenty of apps like Math Drills (read Buzz Garwood's review) for straight-up drill and practice, Alien Equation also involves number sense and spatial reasoning.
Alien Equation has a story behind the game:
When the navigation computer on your interstellar spaceship contracts a nasty virus and reverts to the intelligence of a three year old what do you do? Can you reteach the computer math while fending off the contagious invaders. Rearrange a grid of numbers and operators into enough valid equations before your system shuts down and strands you on a planet full of bipedal, hairless apes.
The backstory actually isn't very important when playing. Playing involves sliding columns of tiles left and right and rows of tiles up and down to line up tiles that make equations. When you start your first game, you are presented with four slides of instructions:
Alien Equation keeps track of progress, adapts to a player's abilities, and allows for isolation of addition, subtraction, multiplication, or division. It's certainly worth the current price of 99¢.