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).