Calc
An extensible, advanced desk calculator and mathematical tool written in Emacs Lisp by Dave Gillespie <daveg@synaptics.com>. Calc runs as part of GNU Emacs. You can use Calc as only a simple four-function calculator, but it also provides additional features including choice of algebraic or RPN (stack-based) entry, logarithms, trigonometric and financial functions, arbitrary precision, complex numbers, vectors, matrices, dates, times, infinities, sets, algebraic simplification, differentiation, and integration.
FTP calc-2.02.tar.z from your nearest GNU archive site.