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)
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