For years I used MathCAD, which is not free of course, but I was able to pick up an older version (MathCAD 6) very cheaply. Unfortunately, I can't get it to run on Windows 7, so I've been looking around at other maths programs.
The two free ones I have at the moment are Octave and Maxima. Both are a bit clumsy in terms of presentation in their own way, but very powerful for computations.
Octave is a command line type program, but once you get past that, its programmable and handles all the good stuff like complex numbers, matrices, calculus etc. Maxima comes with a GUI (wxmaxima) which helps ease the learning curve. For serious number crunching, they're are both good for the money
, but I suspect what you're after is a more for presentation of mathematic equations in documents....? ...in which case neither of these is good (Maxima might be able to be used with a lot of fuss and bother and cut and paste).
There used to be an addin for Microsoft Office for doing equations. I think it was called Mathtype
... I'm not sure now, it doesn't seem to be available standard with Office 2010. I'll stand corrected if wrong.
Al.