During image processing operations such as scanning and scaling, images tend to get blurry. The blurred impression of these images is due to the fact that image pixels are averaged with their neighbors. Blurred images don't have sharp boundaries and look as though they have been taken with an unfocussed camera.
Refocus is a plug-in for the Gimp (the GNU Image Manipulation Program). This plug-in attempts to "refocus" the image, using a technique called FIR Wiener filtering. The traditional technique for sharpening images is to use unsharp masking. Refocus generally produces better results than unsharp masking.
The plug-in comes with a preview that helps you select the best parameters.