During your daily render coding practice, you might need to do some computations, visualizing functions, simplyfing expressions, computing integrals and partial derivatives, displaying debug data and reflected structures.
There's actually one program to rule them all, Mathematica. Geometry Expressions is very nice too. If you don't have and don't want to buy those, you might try a couple of other programs:
GraphCalc - Good graphical calculator for windows, even if most of the times I use the simpler PowerToy calculator or enter directly the expressions into Launchy...
GraphViz - Graph layout
SciLab - Great free replacement for MatLab
Maxima - Probably the best opensource CAS, this is a nice cheatsheet for it
OpenAxiom - Another CAS with a long history
Sage - Uses Maxima for its CAS functionality, adds some more, and presents it in a different sintax
MayaVi - Powerful 3d graphing system
OctaViz - VTK 3d graphing for the opensource MatLab clone octave
Cinderella - Interactive geometry