If you are familiar with MATLAB software, you will find SCILAB fascinating, not just because of its functionalities like MATLAB but because it is an open source software. Open source softwares are free. Here is a small description about SCILAB:
Scilab is an open source, cross-platform numerical computational package and a high-level, numerically orientedprogramming language. It can be used for signal processing, statistical analysis, image enhancement, fluid dynamicssimulations, numerical optimization, and modeling, simulation of explicit and implicit dynamical systems and (if the corresponding toolbox is installed) symbolic manipulations. Scilab is the most complete open source alternative to MATLAB.