Adjusting color temperature: colortemp is a Gimp script for changing the color temperature of images. This script-fu script enables you to change the color temperature of an image to any specified temperature. It can be used to correct the red color cast of photos taken under incandescent light with the camera on daylight setting, or to correct the blue cast of outdoor photos taken in overcast conditions.

Adjusting white balance: whitebalance is a Gimp script for changing the white balance of images. This script-fu script can be used to convert the foreground color to either a neutral gray, or to the color of the background.

Note: I have been told these scripts do not work in Gimp 2.6.0 and 2.6.2 (and perhaps Gimp 2.6.1), due to a bug in the implementation of script-fu: one of the key functions on which the scripts rely has no effect.  It is difficult to provide a workaround, so you are advised to use a later version of Gimp; the scripts work fine in Gimp 2.6.4, which is what I have been using lately.

You can find tutorials on the use of Gimp at the Gimp home page, as well as at Gimp Wiki and at Digi Grey.

Making Photo Galleries with Captions.  makeshow is a Python script that creates a photo gallery web page from a directory containing images, automatically resizing all the images, and creating the thumbnails. The script also creates captions: a caption to a file xyz.jpg can be specified via EXIF, or by including a caption in a file named xyz.txt in the same directory. It requires ImageMagick.