It is a bit surprising that so far no one has mentioned CoCoA (an acronym for Computations in Commutative Algebra), a link to the main site : http://cocoa.dima.unige.it/ where one can download the newest CoCoA 5.0 for major platforms (Linux, MacOS X, Windows), (a helpful manual for CoCoA 4.7 can also be found there, for 5.0 is to be updated).
A freely available computer algebra system (written in C) being developed for over 20 years. Its target are professional, scientific applications, among them Gröbner bases calculations, polynomial factorization, exact linear algebra, Hilbert functions, toric ideals etc.
There are good monographs like Computational Commutative Algebra (2 vol.) by Kreutzer and Robbiano dealing with CoCoA: http://cocoa.dima.unige.it/kr/
In the field of commutative algebra it has more advanced and specialized functionality than Mathematica, although in M there are brilliant capabilities (absent in CoCoA) for quantifier elimination (helpful in dealing with systems of polynomial equations and inequalities) : http://www.wolfram.com/learningcenter/tutorialcollection/AdvancedAlgebra/.