The script enables you to do two image adjustment operations to all images in a folder:
You choose a folder with a folder browsing widget, you can also specify the file extension of the files you would want to change(useful if you have non supported files in the same folder).
You can either create new files or overwrite the old ones. It is not possible to create new folders using Script-Fu, so new files are saved in the same folder, but with a '_' prefix added to the file names.
Unfortunately there is no preview, so you will have to open one of the images in GIMP and find suitable settings for colour balance and brightness/contrast before plotting them into this script.
This is my first script, and my first week as a GIMP user (in other words: I am not a pro!), therefore it might not be perfect, but hopefully it can be of help to someone.
The script appears in File->Create->Misc
UPDATE: Changed the script as suggested by Rob A. in the comment field below. Now it should work on both windows and unix systems
UPDATE: bakalex92 (http://registry.gimp.org/user/23077 ) gave me a code snippet to integrat into the script so that it will work on indexed pictures too (like .gif files), but it should be noted that it does not work on animated images as it only does one layer.
What the script looks like: