Okay, who is using which for what?
PhotoCalc: I'll start by saying thanks to Joseph Ramos for turning me on to this nifty little 99 cent app that has sunrise data and a host of other photo tools.
PCalc lite: Freeware that gives you a basic scientific calculator. Pay for the full one and you get more functionality, RPN options, etc.
TiltMeter Standard: Another piece of freeware that turns your iPhone into a level and clineometer, and is adjustable for zero, a benefit if you use an optional case for it.