I'm in the process of moving from my addiction to Macromedia Fireworks to The GIMP. Why? Well for one thing I really don't have the finances as a lowly uni student to buy new licences all the time, not to mention the uncertain future of the aformentioned program after Macromedia was bought out by Adobe, and the fact that Fireworks isn't a universal binary yet… plus there's that warm fuzzy feeling you get when you use Open Source software ;).
This is how you use the GIMP's Sharpen Filter according to the web based documentation:
This filter is found in Image>Filters/Enhance/Sharpen
Most of digitized images need correction of sharpness. This is due to digitizing process that must chop a colour continuum up in points with slightly different colors: elements thinner than sampling frequency will be averaged into an uniform colour. So sharp borders are rendered a little blurred. The same phenomenon appears when printing colour dots on paper.
The Sharpen filter accentuates edges but also any noise or blemish and it may create noise in graduated colour areas like the sky or a water surface. It competes with the Unsharp Mask filter, which is more sophisticated and renders more natural results.