Recently, on the PIXLS.US forum there was a tutorial by Morgan Hardwood for enhancing shadows.

It entails adding a contrast mask layer (a desaturated, inverted copy of the original image in overlay mode) and then a copy of the original image in lighten-only mode to restrict the effect to the shadows. An appropriate amount of Gaussian blur is then applied to the desaturated layer to achieve the final result.

I made a simple plug-in to create the layers, with the desaturated layer active ready for applying the blur.

I put it here should anyone find it of use. (Requires Python 2.7)

David, Thanks a lot. I tested it yesterday and it works as a charm!

The credit for this must go to Morgan Hardwood. I only made a simple plug-in to create the layers.