Giac/Xcas is a free computer algebra system.
Xcas is an interface to perform computer algebra, function graphs, interactive geometry (2-d and 3-d), spreadsheet and statistics. It may be used as a replacement for high end graphic calculators for example on netbooks (for about the same price as a calculator but with much more performances).
Giac is a free (GPL) C++ library, it is the computation kernel, it may be used inside other C++ programs.
Giac/Xcas has a compatibility mode for people used to the Maple or Mupad CAS as well as for users of TI calculators (TI89, Voyage 200, TI Nspire CAS).
Please see the Installation Instructions for further information.