Python, NumPy, SciPy & Matplotlib
Python is a free programming language that is widely used, most of the software developed for Linux is written in Python. It contains several libraries that cover a lot of problem domains, from asynchronous processing to zip files. Also it is available for most platforms. More information can be found at the language official page.
More specifically to scientific computation, the NumPy project brings n-dimension array objects, random number capabilities, fourier transforms and many other useful tools.
Boosting NumPy capabilities is SciPy, which is another Python library that adds signal processing, optimization and statistical tools to Python.
After all the calculations are done, they can be plotted also using python and another useful library: Matplotlib.