Off the top http://freemat.sourceforge.net/ comes to mind.. There is also http://www.scilab.org/ and https://www.gnu.org/software/octave/
Hmm I wish FreeMat had more documentation, so I can see more of behind the works of how they improved the 3d visualization and n-dimension stuff.
With all three not much on documentation on what I want to see. In the end though it comes with trials, ill just try all of em and see if one is past the parameters of Matlab's functions.
Im pretty sure once I install these programs, I can look through the help section to learn more about certain functions.
Scilab does look good, but from just shifting through some documentation, they seem have some limitations as Matlab does.
On a separate note, do you know how these programs do in the signals and systems area? Curious if its improved from what Matlab has, although Matlab has really strong modules and toolboxes.