This script generates two difference layers from the two layers on the top, A is the original image layer, B the modified (e.g. sharpened) one on top:
A - B = C
B - A = D
A - C + D = B
It's similar to GIMP's built-in Grain Extract/Merge function, but the tonal range is bigger.
Now you can apply further editing and "switch on/off" the differnce by switching the visibility of the subtractive and the additive layer. Also you can change the modification intensity by changing the opacity of these two layers.
Copy it into the scripts folder from GIMP, you will find it then under Layer -> Difference Layers.
Left: B A
Right: D C A B
Finally a big thank you to Shelly and Roy from the Flickr GIMP group:
Update1: I renamed the script from "Adjustment" to "Difference", which describes the script's function better.
For all scripts together, go here: