any creative math apps you could recommend?

[SarfEast1cated]

Hi there all, I am on my PGCE placement, so not really a teacher yet but please don't kick me out of the staffroom!
I am looking for a good app for my Y2 class to help them with their number squares, some thing to really drive home one more, one less, counting in 3's etc. We do them on the IWB, but I think if they practiced an ipad it might be more fun for them. Can you recommend anything you use for this?
We use teach your monster to read on the computer, and they all love it, so something as equally engaging and fun would be great...

My DDs' school uses SumDog. They love it. The teacher can set the work/topic/range available to the children, monitor what they do at home and limit time (for example if parents find that their child gets obsessed with it at home).