The one I used to use was Beer Alchemy. Pretty good app though for some reason I could never get the numbers the way they should be for my equipment. Not necessarily an issue with the app as I'm willing to believe it was operator error.
I since moved to Beersmith on my computer so don't use the Beer alchemy iPad app anymore. Someone said Beersmith has some stripped down app on the iPad now while they work on a more robust version but I haven't tried it. I guess you can't create recipes with it, but it does load the recipes you made on the computer version and has a bunch of calculators so could be worth checking out.