GIMP Plugin Registry

Uses one colour for the highlights and another for the shadows.

This Shadow&Highlight script does a decent job of lightening shadows and/or darken highlights in photographs. The UI gives you five control sliders. Two for control of the Shadow and Highlight amount (opacity), two for the tonal width of each contrast mask layer (using gamma), and one for the blur radius of the masks. Plus the option to flatten visible layers when done.  read more »

This plugin does a pretty good job of eliminating shadows and highlights in photographs. It gives you two layers that basically act like adjustment layers, and changing the opacity will alter the effect.