As a RPN calculator user, my first choice is XCalc (http://www.tordivel.no/xcalc/ http://www.tordivel.no/xcalc/). The non-RPN one that I use the most is SpeedC

As a RPN calculator user, my first choice is XCalc (http://www.tordivel.no/xcalc/ http://www.tordivel.no/xcalc/). The non-RPN one that I use the most is SpeedCrunch (http://speedcrunch.org/ http://speedcrunch.org/), Insect (https://github.com/sharkdp/insect/ https://github.com/sharkdp/insect/) and Frink (http://frinklang.org/ http://frinklang.org/). All have unit conversion too.