Sage: A free open-source alternative to leading commercial math packages. You can download it to your computer or you can use the Drake Sage server (please see me to get an account).
Octave: Open-source free alternative to Matlab. You can download versions for Windows, Mac, or Linux.
Mathematica: A comprehensive computer algebra system. I think we have a campus license, so you can use it for free while you are here. Student licenses are not valid after you graduate, so student versions are basically “rented” until you graduate. Depending on what you do when you leave, Mathematica may cost thousands of dollars after you graduate.