PCalc is really the best, functional and customisable.
I live in the Netherlands which has VAT tax called “BTW”.
In the business world you often find yourself needing to find the BTW (tax) amount for something, or having a cost excluding BTW, and wanting the amount including BTW, or, even, having an amount including BTW, but you need to calculate the amount before BTW was added.
With PCalc I can make a customised keyboard with named keys (BTW, BTW+, BTW-) which call PCalc financial functions and calculate any one of these with one key.
Maybe a simple thing... but very handy for day to day tasks.