On my PC desktop I use an RPN calculator, I find it so much more useful than a standard style calculator. I think it's because you can see the stack and visually validate the numbers you are working on.
I use RPN Calculator v1.3 by Bill Menees, I find the interface a lot cleaner that the others I've tried.
At least in my experience, it allows easier reuse of previous values. This makes RPN based calculators easier to use for exploratory kinds of work.