In a Mashable post about HTML5,   I found MathBoard, from Pala Software. "MathBoard is a popular education app ... that helps students learn and practice their integers."

"MathBoard Addition is appropriate for all ages from kindergarten to elementary school where learning math can be a challenge. You can control the range of numbers you want to work with, the number of questions you want to answer and even assign a time limit per quiz. MathBoard Addition will make learning math fun. "

Mathboard is available as
a free Google Chrome App (addition only)
a paid Google Chrome app 3.99 US$ (Addition, Subtraction, Multiplication, Division, Squares, Cubes and Square Roots)
for the iPad (US$ 4.99) (available in English, Dutch, French, German, Japanese, Portuguese)
software for Mac 10.6-6 or later

Here are two videos about the iPad version: