Basically, we haven't discussed it much yet but the Curves adjustment tool is a great way to adjust tone and color (and therefore exposure) ranges throughout a photo all in one convenient place. Very useful. See Photoshop menu Image --> Adjustments --> Curves or for similar processes in GIMP try menu Colors --> Curves.
There's an extremely clear and understandable tutorial about how to use Curves over at Tutorial9.net
Curves can be overdone with great ease... but they can also be a convenient way to tweak a lot of color and value settings all at once. And if you want very subtle control using layers options like 'lighten' or 'darken' or 'luminosity' or 'hue' with some transparencies too, try using Curves in a new Layer, or Adjustment Layer. (actually the same is true with any of the processing controls we've tried throughout this class).
Visual example of the differences... using curves to fix the tonal range in the shadows and midtones...
