I've been trying to learn how to use 'curves tool' on GIMP. It is very useful tool for adjusting the contrast and brightness selectively in very similar way as one would use sound equalizer on music player. With extreme adjustments it can been done such things as 'solarization' aka Sabatier effect. Less dramating editing can be done to make the dark areas lighter and vice versa. But the best thing about curves tool is that it is fun.