usm

This script sharpens an image with the built-in GIMP plugins erode and dilate.
These two plugins infiltrate the image with the lowest/highest pixel of the closer environment, but never higher or lower.
http://docs.gimp.org/en/plug-in-dilate.html
http://docs.gimp.org/en/plug-in-erode.html

This means, they can be used to sharpen an image w/out halos.