Computer Algebra Systems
- a free version called Maxima is available, but I cannot vouch for it.
Derive - a good "algebraic calculator" - now owned by Texas Instruments - discontinued 2006
Maple - up to date system written in C++ and well supported, great graphics
Mathematica - similar to Maple but more features and wider support with better graphics
Reduce - an old favourite I used on a mainframe years ago, it is still available for a PC
Axiom - this system is interesting, I want to know if it is any good - discontinued 2001 - passed to open-source consortium
MuPAD and MathCAD - not as heavy weight as Maple or Mathematica - never seen them used
Numerical Systems
MATLAB - simply the best, does advanced number crunching and graphics very efficiently and allows access to the vast number of routines included. Only low level functions are precompiled.
There are still situations where I use Basic, Fortran and C, but rarely. To get decent graphics you also need an additional graphics library.