RPL/2 Programming Language - Forth with a hint of Lisp of HP fame

Of HP fame is a bit missleading. HP calculators are programmed with the RPL Programming language,[1] that was both a high level user language and a low level system language for the 4-bit Saturn processor.

This is a free and portable reimplementation, with a few extensions (and some incompatibilities).

There also is a unrelated iPhone RPL reimplementation [2].

IMO the most comfortable way to program a calculator, i personally use a HP-28c with 680 KHz 4 bit cpu. I think it's crazy how much you can do with so little power.

[1] http://en.wikipedia.org/wiki/RPL_(programming_language)

[2] http://naivedesign.com/ND0/ND0.html