GIMP Plugin Registry

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 »

Deskew, also called auto straighten, is the automatic rotation of an image such that the text is vertically aligned. This is great for cleaning up scanned documents.

http://www.cubewano.org/gimp-deskew-plugin/

Implement several different black and white conversions including color filters

More information:

http://epaperpress.com/psphoto/
http://www.thelightsrightstudio.com