I've got multiple calculator apps on my phone. And some on my computers. And physical calculators. Sometimes it's nice to not need to swap what's on screen, and instead just swap to a different device. Definitely worth having a good phone app or two (or eight, maybe I have too many) for convenience though.
RealCalc (most used, it's a very nice RPN scientific calculator with good unit conversion capabilities)
TI-89 emulator Graph89 (with an RPN application)
BitCalculator (handy for bitwise calculations, can show all 64 bits of a value at once, signed/unsigned integers, 8/16/32/64-bit widths, big/little endian, and IEEE 754 single & double precision floating point)
WolframAlpha (online-only, but very powerful)
ConvertPad (dedicated unit conversion calculator)
HP-48 emulator Droid48 (replaced by RealCalc, never deleted. Familiar button layout)
Electrodoc Pro (Electrical & electronics calculator & reference app)
Slide rule emulator, for silliness. If you can't pull out your phone & use it as a slide rule, are you really calculating?
I've got two physical TI-89s, a TI-36x solar, and an HP-48.