I think Brian is initially looking for presentation / rendering (correct me if I'm wrong Brian) - in which case OpenOffice is really okay. I have used OpenOffice for the expression of mathematics for several years and it's quite expressive - the language used is generally quite simple but there is a learning curve with anything. I have printed the manual for the OpenMath component and still have to refer to it on occasion - so I keep it by my desk at home. When ever I write notes, commit thoughts to electronic form, I rely upon OpenOffice. For publication purposes LaTeX appears to be the language of choice but I have not tried LaTeX - OpenOffice works for my own private writings.
If you seek something more like computational assistance for purpose of learning then Bojan makes some very good comments - there are pitfalls (and clever work-a-rounds) that only proper training will generally provide.
The three mathematical packages worth considering (in my opinion) are
wxMaxima - focus: computer aided algebra
scilab - focus: numerical processing
R - focus: statistical processing
All three packages are free and there is considerable overlap in their capabilities.
And while off-topic I'd like to also add
TopCat as a very useful tool for astronomers who wish to process data from various tables and astronomical catalogues. I only include this as these four packages - from my reading - as they more or less cover anything a serious amateur astronomer is likely to require without having to write dedicated software or perform some quite intricate manual mathematics.
Caveat: I don't have any meaningful experience with these packages but they are the packages that seem to emerge as the free resources that are pre-eminent for professionals working with science data (from what I perceive). These are the packages that my explorations suggest that I should, at least, acquaint myself with.
And of course
OpenOffice - I'd recommend it to anyone for general "office productivity" requirements.
Mark C.