I like to browse the registry for scripts occasionally and I found this interesting script. It does a nice job. I recommend going through the tutorial that's linked in there; it will help you to know about the settings in the script dialog. It is NOT a tutorial using the script but one that demonstrates what the script does and how the script writer arrived at making the script.
It's similar to Unsharp Mask and in some instances may be superior.
http://registry.gimp.org/node/24876Here are before and after images. I may have set the sharpen settings a bit high. Image found
hereBefore
After
In GIMP it is in Filters > Eg > Sharpen (Smart Separate Redux)