Most of us have the calculator feature on the computer, but on a lot of occasions, I have found myself wishing there was a calculator right on d2jsp so I didn't have to swap windows, etc.

I'm not sure what exactly it would take to get this going, but it seems relatively simple. Just a thought.

No for me, as you said, every operating system of every computer has a good calculator program.
And where would the calculator be? Wouldn't you need to switch pages/windows either way?

No.
There is no need for such feature.