GNU Image Manipulation Program, a sort-of free Photoshop. [1 ]
Note: In the same area, we should keep an eye on the upcoming versions of tcl-magick, a Tcl interface to the ImageMagick tools/library. For complex image manipulations, it won't have the power of GIMP though (IMHO of course).
JOB Here is a small shell script example, which uses the script-fu extension: