Free iPad app: http://itunes.apple.com/us/app/spelling-test/id485279878?ls=1&mt=8&uo=4&at=10l6SU
Free iPhone app: http://itunes.apple.com/us/app/spelling-test/id485279878?ls=1&mt=8&uo=4&at=10l6SU
Paid iPad app: http://itunes.apple.com/us/app/vocab-a-splat/id517271626?ls=1&mt=8&uo=4&at=10l6SU