Oh, and CreativeCreek's excellent MathU RPN on the iPhone, which feels insanely HP-like as it mashes together the functionality of several HP calculators in a very useful way. (It does the work of the 12C, 15C, and 16C quite well - at least all of what I use on each!)

MathU remains one of only three apps I've ever paid for. It is actually the last major thing keeping me from leaving the Apple ecosystem, since there's no Android version. Yes, it's that good.