This script is a filter trying to imitate the effect of Clarity slider. This is in essence USM (Unsharp Mask) masked to be effective only in midtones. For best results try large radius values (100 or above).
As currently file uploads don't seem to work, download from (remove spaces):
sourceforge . net /projects/johnlakgimp/files/Clarity/clarity_filter_v1.0.zip/download
Copy the .scm file into:
Windows XP and older:
c:\Documents and Settings\[USER]\.gimp-2.8\scripts