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.
Every paper size has a different shape, and none probably match the shape of the images produced by your digital camera. This little utility will create a layer showing which parts of an image will fit on the paper when printed. A wide selection of print sizes are included. Using it will help you layout your image so that important pieces aren't cut off by the printer/lab.
This script-fu was developed under Gimp-2.0 and Gimp-2.4