Speaking of calculators, I once came across this gem and then inexplicably forgot all about it. It's called orpie and it aptly dubs itself the mutt of calculators. It's RPN, it's curses, it's at least as rich and powerful as your HP 48G (in features, that is), and it's completely configurable a la mutt.

A few standard tricks to get it compiled on OS X:

sudo port install ocaml gsm
export LDFLAGS=-L/opt/local/lib/
export CPPFLAGS=-I/opt/local/include/
./configure
make
sudo make install

I set up iTerm to run orpie in a new tab on ctrl-cmd-o, to ensure that I never forget about this powerful calculator again. Go ahead and give it a try. It's like an HP 48G, but unlike x48 and other emulators, it's actually usable from the computer keyboard.