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.

Copy it into the scripts folder from GIMP, you will find it then under Filters -> Enhance:
http://docs.gimp.org/2.6/en/install-script-fu.html

Example:


Update1: Added Antialiasing.

If you like it, hug someone.