For me there are certain mental calculations that take longer to process than others. 2 + 2 is super easy. 7+8 is not as immediate. I need a program like this that learns which calculation take me longer, even if it's just slightly longer, and then drills and drills my weak points over and over again. That would be a real mental math trainer.
With a bit of tweaking, Anki [0] or another Spaced Repetition Software program will do just that. You can configure it to categorize your answers by how fast you answered them. Then it will make the easy ones less frequent.
I used it to memorize the times table up to 20x20.
