Simulate HDR is a plugin for The GIMP that creates two layers from a single image simulating an underexposed and a overexposed shots, creating a High Dynamic Range image.
It can create a tone mapping layer to compress the color range of the image.
A Gimpplugin that implements the Octave Sharpening method
Octave sharpening is a plugin for The Gimp that implements a method which produces intense sharpened images without halos or noise.
If a High Pass Filter is not enough, try this.