gdcalc

gdcalc is a financial, statistics, scientific and programmers
calculator for Unix. The GUI was written with glade(1) and uses the
Gnome/Gtk+ toolkit - so it may well be compatible with themes and
other whiz-bang features of those systems.

gdcalc has both Algebraic notation (ie. conventional, TI or Casio
style) and Reverse Polish Notation (RPN or Hewlett-Packard style).
If you've not heard of RPN before, you are probably familiar with
algebraic calculators. Very briefly, while simpler and more natural
to use, RPN calculators need some study eg. they have an Enter key
instead of the equals key.