I started using Forth because I wanted a Reverse Polish Notation calculator at the command line. It turned out to be very practical and extensible.

For example, I created many words to perform unit conversion, such as temperature:

    : 2fahrenheit 9 * 5 / 32 + ;
    : 2celsius    32 - 5 * 9 / ;

Furthermore, I can even pull in live currency conversion rates from Yahoo Finance to go between USD and CAD.

Since my underlying Forth interpreter runs on Python[0], I get the full large integer and floating-point support that Python has.

[0] http://openbookproject.net/py4fun/forth/forth.html