Hi Ben,
Though I don't normally use apps when performing - the best app hands down to fool magicians (and laypeople) with is called Mental Killer by Max Krause. You should probably have an iPhone (I have a i4, but any iPhone will do), but doing it with an iTouch would work too.
Anyway, it's so good, that it's basically like carrying an 'out' with you 24/7.
iForce by Greg Rostami isn't bad either.
Good luck,
-Sanj