dc on the unix command line (linux too), is an rpn calculator. I use its non rpn counterpart bc. I've been told they aren't great (buggy), but for simple math they work fine.

I used a hp 32S in college, less mistakes make with rpn then entering calculations the other way. though newer calculator allow entering of whole equations ((10 *5)^2)/3 made this less useful.

The hand helds are more portable than emacs.