I teach geometry, calculus, and differential equations to high school
students.
One area that many students have difficulty with is visualizing 3D
objects.
I have used mathematica successfully in these classes.
It is really useful to be able to graph intersecting planes, planes that
are tangent to spheres, surfaces of revolution, etc.
Being able to plot in 3D is incredibly helpful in understanding partial
derivatives.
To be able to plot not only functions of z but parametric plots and
implicit plots in mathematica make it incredible versatile but its very
expensive.
The problems I have been having with maxima; its apparent limitations.
Not being able to plot z as a constant function (at least under windows
XP since apparenly you can under linux) and particularly not being able
to plot several surfaces in one plot make maxima pretty useless in this
regard.
For many of you perhaps using Maxima as a tool for plotting is not
important, but I hope I can encourage those of you who are developing it
to enhance its capabilities in this area as a boon to mathematical
education. To have a free program that could do these things would be
fantasticly useful to high school teachers.