>>Button+dg
I run a TI 83+ emulator on my Android phone when I don't have my physical calculator at hand. Same concept, just learned a different brand of calculators.
>>Vorpal+Th
built-in calculator apps are surprisingly underbaked... I'm surprised neither of the big two operating systems have elected to ship something comparable to a real calculator built in. It would be nice if we could preview the whole expression as we type it..