[fbriff]

As a RPN calculator user, my first choice is XCalc (http://www.tordivel.no/xcalc/).

The non-RPN one that I use the most is SpeedCrunch (http://speedcrunch.org/), Insect (https://github.com/sharkdp/insect/) and Frink (http://frinklang.org/). All have unit conversion too.

[severak_cz]

Frink is my tool of choice for computing, despite it bit esoteric.