Free MATLAB Substitute

During my PhD days, I had looked around a lot for Matlab substitutes. While MATLAB is an excellent piece of software, it is prohibitively expensive. Two substitutes I can recall immediately are Octave and Scilab. While GNU has generally cut it well on the software it offers, somehow Octave has not made it. I definitely appreciate the time the community has put in; my comment is based on the conversations I have had with friends. Perhaps the non availability of an easy to install MS Windoze version may have something to do with it. My main requirement is image processing and vision. There are tonnes of software written in MATLAB, which is perhaps the main software of choice in many universities. So, a MATLAB substitute must be able to process this vast collection of tools with no or minimum effort. Sadly, this may not be possible. One problem with such packages is that these get tied down to specific releases of MATLAB.
Recently, I came across FreeMat which seems to be a good substitute. I have barely begun to use it but some people I know report a fairly good compatibility with MATLAB. At least the general commands seem to match well. The flip side is that compiling from source in GNU/Linux is a bit of bear.