if it can be done with a $1 calculator you probably should have done it in your head
in circuits if I need more accuracy I usually have gone to the PC loaded with free SciLab/Octave and LTSpice - which is a "engineer's Spice" - worth learning the quirks
some are now claiming Python SciPy, NumPy and SymPy in iPython Notebook are a replacement for many other specialized math sw tools
http://nbviewer.ipython.org/github/jrjohansson/scientific-python-lectures/blob/master/Lecture-0-Scientific-Computing-with-Python.ipynb but I don't think SymPy is really there yet - Macsyma/Maxima is available in Sage Math - but Sage runs in VirtualBox on windows - wxMaxima can work without the VirtualBox
other free Computer Algebra: Giac/XCAS which is used as the symbolic math engine in GeoGebra - GeoGebra dynamic geometry environment is actually quite powerful despite the primary/high school math orientation - and you can import datasheet graphs as images and then do geometric constructions/math right on top of the image
paid math sw get pricey quickly but is often worth it to your company if the sw features save months of engineer's time
MatLab, Maple, Wolfram Mathematica all go way beyond the freeware offerings - particularly in specialized packages for signal processing, control, image analysis...
I used to use Mathcad - version 11 was the best for me, the last with the Maple symbolic core - but it changed to muPad - I never transitioned to the later versions as it broke my existing symbolic worksheets