I just saw Pi Cubed, and now I'm thinking, what Math/Calculator apps are available for iPod Touch?
I use wolfram alpha. its a great app for computing basically anything.
If you love the old-school reverse-polish-notation (RPN) HP calculators, like I do, then check out i41CX. It's an iOS emulator for the HP-41CX calculator, and is very faithful (except for having way more memory than the original): for example, synthetic programming is supported. I have not found an HP-48 emulator for iOS yet.
I have i41CX+ installed on my (second-generation) iPod Touch. It's probably overkill for the uses I have for a calculator these days, but it works the way I expect a calculator to work.
I use (and rate) Missing Calculator, although by the looks of things it isn't in the App store (UK) any more..