highpass

Yet another high pass. I use it to normalize brightness of textures. This script provides two different approaches: grain extract mode (mainly used to remove the effect of vignettes) and soft light mode (for images with sharp lightness transitions, but this mode is not so powerful as grain extract).

After installing, you'll find it in:
Filters/Enhance/High Pass Filter

Result:

Highpass sharpening script (this is my first script for Gimp).
This technique is described by Rolf Steinort in episode 164 of the Meet the Gimp! video podcast.
You can run it from: Filters -> SkYNeT -> Enhance -> High Pass Sharpening
Because script makes a new layer from visible, make sure you have visible only layer you wish to sharpen. Other layers must be non visible.

A general purpose high-pass filter plugin. It shows up as Filters->Generic->High Pass Filter. You can select a blur radius that sets the size of detail to be passed by the filter, as well as an option to keep the source layer or replace it.

EDIT: Updated to use grain extract rather than the previous method, This looses less histogram data and is closer to the Photoshop version...

It actually implements 5 different high pass modes:

Implement a highpass filter sharpening

Infos about highpass sharpening:
http://www.gimp.org/tutorials/Sketch_Effect/
http://www.retouchpro.com/forums/showthread.php?s=&threadid=3844&highlig...

Updated to work with Gimp 2.8.