Yacas (Yet Another Computer Algebra System) is a small and highly flexible
general-purpose computer algebra language. The syntax uses a infix-operator
grammar parser. The distribution contains a small library of mathematical
functions, but its real strength is in the language in which you can easily
write your own symbolic manipulation algorithms. The core engine supports
arbitrary precision arithmetic (although it can optionally be linked with
the library "libgmp3") and is able to execute symbolic manipulations on
various mathematical objects by following user-defined rules.

WWW: http://yacas.sourceforge.net/