Computer Algebra System
A computer algebra system (CAS) is a software program that can perform symbolic computations, such as expansion, factorization and other operations of polynomials, differentiation and integration of functions.
Such facilities are usually integrated with other utilities such as graphing, data handling and word processing tools to provide comprehensive packages for professional uses as well as teaching and learning. CAS can be run in common computers as well as some hand-held platforms. In particular, calculators equipped with CAS are posing challenges to our assumption of basic algebraic skills, similar to the impact of scientific calculators on the notion of essential computational skills.
See introduction to CAS at Wikipedia.
Some popular software programs with CAS:
TI InterActive!  
Scientific Notebook and Scientific WorkPlace
Eigenmath (freeware)
Here is a page of Eigenmath Introductory Manual.

Examples of calculators with CAS:
Voyage 200, TI-89 from Texas Instruments
Classpad 300 from Casio

Examples of CAS tools on the web:
Integrator  (symbolic integration powered by Mathematica)