The ever popular Add-A-Number pad has been adapted before to calculators, usually at quite a cost. Paul Daniels has come up with an iPhone / iPod / iPod app which is very good. The spectator can turn on your iDevice and click on the calcutor, then pick three numbers to multiply together. When they hit the equals button, the answer is forced. The total can refer to a page number and word in a book you predicted, the serial number on a bill, anything you want.
You never touch the device yourself. You can force three different njumbers (of your choosing). Or you can have the calculator revert to its normal, ungimmicked self so the spec can punch in numbers to his or her heart's content.
It's very good, I think.
It's 0.69 in the UK which is $1.00, I think, in the U.S.
It's "Paul Daniels Calculator" in the iTunes store.
(I don't work for Paul, or anybody, come to think of it!)