Stacniac is a slightly odd RPN calculator that doesn't have buttons: you enter numbers and then type in commands. Apparently, most people prefer algebraic calculators and like to click on buttons, so I don't think Stacniac will have wide appeal; however, I like RPN and prefer to run the calculator from the keyboard. Stacniac has a fairly large set of commands which are invoked by typing short (but hopefully obvious) words. Stacniac is programmable in a limited way. It also has engineering, hexadecimal and currency display formats (which are customizable).
Mac OS X 10.1
||Mac OS X 10.1 or later