Mupad is a GUI driven MATLAB package that helps you do algebra, calculus, as well as to graph and visualize functions. As you know, MATLAB is good for writing simple programs and working with numbers, but is cumbersome for doing symbolic calculations. In contrast, Mupad works with symbols by default, and has a nice menu-driven interface
There are many different computer algebra packages available, with different strengths, costs, etc. Maple, Mathematica and Matlab are among the best known commercial packages. Sage builds (among other things) upon Maxima, and has the distinct advantage of being open source (and free as in thought, and free as in beer). Personally, I am trying to work largely with Sage (check out sagemath.org), although I know others who prefer one of the commercial packages.