SCILAB software is available for windows, mac and linux. It is a numerical computation tool and opensource alternative to MATLAB. It can be downloaded from http://www.scilab.org/.
Excellent tutorial on SCILAB are following:
- Introduction to SCILAB
- MATLAB-SCILAB Equivalent
- Matrix Structural Analysis of Plane Frames using SCILAB
The source code of above msa scilab document is available here. It can analyze 2D beam, frame and truss.