This technique gives similar results to the Tone Mapper adjustment in LightZone. It reduces global contrast in images while increasing local contrast and shadow/highlight detail. Use this to make your images "pop."
Load your image up in Gimp and follow along...
1) Duplicate the layer twice:
2) Desaturate the new top layer:
3) Invert the top layer. It should look something like this:
4) Blur the top layer (radius of 100 - 500 is good; the smaller the radius, the stronger the effect):
5) Set the blurred layer opacity to 75%:
6) Merge down the blurred layer for a tone map adjustment layer:
Here's what the merged layer will look like:
7) Set the new layer to "Soft Light" and adjust the opacity until you get the result you like (90% is a good starting point):
Here's the end result. Notice the additional detail in the street, tree, and horse's neck; also the bright/hazy region in the top left corner is improved a bit (Mouse over for original).