http://pari.math.u-bordeaux.fr/

PARI/GP is a computer algebra system designed for fast computations in number theory, but also contains a large number of other useful functions to compute with mathematical entities such as complex numbers, matrices, polynomials, power series, algebraic numbers etc., and a lot of transcendental functions.

Short presentation in French: http://logiciels-libres-cndp.ac-versailles.fr/article.php3?id_article=56