MATLAB is great, but not free and not open-source.
You might be interested by Free and Open-Source clones of MATLAB or MATLAB-like softwares? This page is here for this.
GNU Octave aims to be a full clone of MATLAB, and has been developed for the last 25 years. It works really well, and now has a built-in GUI, very similar to the MATLAB one.
SciLab, was originally developed by a French team, joint work of CNRS and INRIA. It has a syntax very close to the MATLAB and Octave one, and is still actively developed.
More details on their website: SageMath.org.
Python with the Spyder IDE (with IPython), the NumPy/SciPy packages for computations and MatPlotLib for plotting.
All this is included in the free Anaconda installer, which allow to install everything in two clicks (one huge file of 400 MB, free to download of course).
Note
"Freeware"?
Each of the mentioned softwares are freely available, on Windows, most of the Linux distributions, or Mac OS.
For more details, and more alternatives (e.g., more specialized alternatives), you can for example refer to these pages: