R -- I've heard good things about R, and it's a good tool for certain fields. But if the original poster is interested in going into an Engineering field then different tools are more appropriate (It's better to use the same or similar tools that your colleagues are using).
Matlab -- one of the industry standards for number crunching and plotting.
Octave -- a Matlab clone that is frequently good enough. (My company can't quite afford a Matlab license, and I get by with Octave when I need that functionality).
Maxima -- for CAS, I haven't used it much, I've heard that it works.
Python with NumPy, SciPy, and Matplotlib -- Highly recommended. The combination is almost as good as Matlab for number crunching, and it is more generically useful.
Orpie -- great command line calculator