GIMP Plugin Registry

This script creates small rectangular selections and auto-white-balances them.
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Light and Shadow -> White Balance Puzzle.
Be careful with the feather option, it consumes a lot of cpu power.
This script gives a strange result, so it just fits in rare conditions:  read more »

To fix burned out or white skies by adding clouds and a blue sky gradient (using a threshold mask).

Guide to Using the Blue Sky & Clouds Script

To fix burned out or white skies by adding a blue sky gradient (using a threshold mask).

Guide to Using the Blue Sky Gradient Script

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.

Implement several different black and white conversions including color filters

More information:

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