This is a very simple script that adds the rule of thirds guides to the current image.
I used to add guide by guide by percent, then crop, remove the guides and add them again one by one. After many times I did it like that, I decided to create a script to do it faster, for which I had to self train on how to do it using script-fu, to avoid poor portability.
I created the script with my Fedora 8 Linux distribution for GIMP 2.4, but it also works for 2.6.
This script works for any platform that GIMP works with.