Mac OS X already comes with a decent calculator, but if you’re looking for something a bit more ambitious, PCalc fits the bill.
This dashkard covers all the shortcuts for switching modes, making calculations, editing and more. I only left out the most basic ones — I assume you’ll be able to figure out how to enter a digit on your own 
As always, this new dashkard will appear automagically in the dashkards widget.
Tip: PCalc is also available for iOS, by the way.
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