GNU Image Manipulation Program, a sort-of free Photoshop. [L1 ]
CM (28 Jul 2003): In addition to the official scripting language of GIMP (i.e., script-fu, a dialect of Scheme), there are also:
http://www.goof.com/pcg/marc/gimp.html
http://www.daa.com.au/~james/software/pygimp/
http://cvs.gnome.org/lxr/source/gimp-plugins-unstable/gimptcl/
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).