These apps are helpful for skill practice.
Free iPad app: http://itunes.apple.com/in/app/tiny-chicken-learns-math/id490114227?mt=8&uo=4&at=10l6SU
Free iPhone app: http://itunes.apple.com/in/app/tiny-chicken-learns-math/id490114227?mt=8&uo=4&at=10l6SU