Script-Fu -> Auto Contrast
FU_contrast_auto-contrast.scm
Automatically adjusts contrast of the current drawable by duplicating the layer, setting the new layer to Value blend mode, then running Auto Levels on the Value layer.
If "Merge layer" is left unchecked, you can open the Layers dialog and adjust opacity of the newly created layer, effectively lessening the amount of change it has made to the image.
Auto Contrast Dialog
Original Image
Auto Contrast Result:
