I would recommend Scilab as I have used it and found it to be a decent open-source alternate for MATLAB. It doesn't have the robustness and polish of a professional package, but since it's based on the MATLAB language, what you'll learn can be transferred later on if your needs change, or you find yourself working in an environment where MATLAB is the default.
Scilab is to MATLAB as OpenOffice is to MS Office. That is to say, it's a not-quite-a-clone, and it's not as polished. You do get most of the functionality of MATLAB, and the price is much more agreeable.