This script simulates a photochrom image, a lithographic printing process from the 1890's:
http://en.wikipedia.org/wiki/Photochrom
The script is a translation of this how-to for PSE with some improvements:
http://www.alibony.com/pse/031508postcard.html
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Artistic -> Photochrom.

If you like it, hug someone.
An example can be found here:
http://www.flickr.com/photos/28653536@N07/4390570531/

Update1: Slightly different contrast layers. New retro option which replaces the orange with a yellowish color.
It's recommended to switch layers on for further edits (ctrl-L). Example here:
http://www.flickr.com/photos/28653536@N07/4394791509/

Update2: Optional b/w dodging. It also deactivates the extra color layer. Thx to Francois for the tip.
Update3: Added a batch version of this script, run it with
gimp -i -b '(elsamuko-photochrom-batch "picture.jpg" (list 255 128 0) (list 255 68 112) 60 60 0 100 FALSE FALSE)' -b '(gimp-quit 0)'
or for more than one picture
gimp -i -b '(elsamuko-photochrom-batch "*.jpg" (list 255 128 0) (list 255 68 112) 60 60 0 100 FALSE FALSE)' -b '(gimp-quit 0)'
Be careful, the batch script overwrites the original image. More info here:
http://www.gimp.org/tutorials/Basic_Batch/

For all scripts together, go here: http://sites.google.com/site/elsamuko/gimp