This is a vague question.

Maxima (computer algebra)

Derive (computer algebra - being phase out in favour of Maxima)

Matlab (numerical computation if I have no option)

FreeMat (as a substitute for Matlab on my computers)

Euler (numerical computation and presentation graphs)

Python (numpy/scipy, testing to see if can replace Euler)

C (for embeded DSP)

OpenOffice (drawing diagrams)

LyX/LaTeX (mathematical report writing)

Gnumeric (statistical/numeric spreadsheet modelling)

PaintShopPro v4 (preparing illurstrations for reports)

PDF viewers (viewing the output of LyX/LaTeX)

GhostScript/GhostView (viewing the output of LyX/LaTeX and manipulating eps)

CB