This plugin provides a simple interface between GIMP and GNU Octave.
Means you can edit a picture from GIMP in the high level numerical language Octave:
http://www.gnu.org/software/octave/

Download elsamuko-gimp-octave.c and install it with:
$ gimptool-2.0 --install elsamuko-gimp-octave.c
or download elsamuko-gimp-octave.tar.gz and install it with:
$ tar -xzf elsamuko-gimp-octave.tar.gz
$ cd elsamuko-gimp-octave
$ make

The second one is also able to use OctaveEmbedded instead of the Linux system call, you have to change the
#define USE_OCTAVE_EMBEDDED FALSE
in the file elsamuko-gimp-octave.cpp at line 48 to TRUE before compiling with make. But since OctaveEmbedded is not reentrant safe yet, it is recommended to use the system call.

You will find it then under Filters -> Mathematics -> GIMP Octave.
The input/output matrices and the Octave script are stored then in ~/.gimp-octave.

For more interesting examples, download and unzip the filter pack in ~/.gimp-octave.
The filter pack needs the octave-image package which should be available in your repository.
These are from here:
http://www.csse.uwa.edu.au/~pk/Research/MatlabFns/index.html

Søren Hauberg modified the plugin to a version with included terminal:
http://image.diku.dk/hauberg/gimp-octave-terminal/

For example in this image the red layer has been fft-shifted by Octave (needs 5-6 secs for a 1MP image):


Update1: Some IO fixes, OctaveEmbedded and possibility of using S/W images.
Update2: README added, some cleanup and a more general makefile in the tar packet. Default Octave script can do backup files optionally.
Update3: Some internal fixes && clean-up.