High pass filter is a easy and effective mode to sharpen an image. Yes there is Unsharp Mask... filter which in spite of its name is sharpening the image [you can find under Filters>Enhance>Unsharp Mask...] but what I like about High Pass Filter is that is a non destructive filter - create a new layer for the sharpening effect.
First this filter is not into your GIMP install and have to be downloaded separately from here:
What you just downloades is a .scm type of file and placed into the scripts folder:
program Files>GIMP 2>share>gimp>2.0>scripts
If you have GIMP running go to Filters>Scripts-Fu>Refresh Scripts to get the filters in your menu.
The process of sharpening is simple just by 2 steps:
1.Activate the filter from Filters>Generic>High Pass Filter. It will create a new almost monochrome layer of your image
2.In the Layers Mode change normal with Hard Light from pull down menu.
After the High Pass filter:
If you fell the effest is too strong insted of Hard lighr layer mode you can choose Soft light.