Use a High Pass Filter to Improve the Detail in Your Photos.
This tutorial is for GIMP.
First of all you need to install high-pass.scm. Download high-pass.scm and put it in your scripts folder. On my computer its C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts. You'll probably see a lot of other .scm files in there. To use it, just select it from the menu: Image window -> Filters -> Generic -> High Pass Filter.Then put the layer on overlay or soft light or hard light.
Download high-pass.scm HERE