Finn's Octave Page
Octave is a free software tool for numerical analysis and visualization. The function and command syntax is very similar to MATLAB. Many contributed functions packages (like the toolboxes in MATLAB) are available. They cover control theory, signal processing, simulation, statistics etc. They are installed automatically when you install Octave.
There is no SIMULINK-like tool in Octave, but there are many simulation functions (as in Control System Toolbox in MATLAB). Octave is available for Windows and Linux.