The wavelet decomposition of an image results in multiple images
with different frequency content. When amplifying the high frequency
parts the recomposed image appears to be sharper than the original
one. That way the frequency which should be amplified most can also
be selected and a given unsharpness in the original image can be
taken into account.

The wavelet sharpen plugin enhances apparent sharpness of an image
by increasing contrast in high frequency space. The amount of
unsharpness of the original image can be taken into account by
adjusting the sharpening radius. As an option you can choose to
sharpen the luminance (YCbCr) channel of the image only.

The plugin can be found under "Filters > Enhance > Wavelet sharpen"

WWW: http://registry.gimp.org/node/9836