[updated 11/28/2011 be 2.7 compatible- A HUGE thank you to François Collard for providing the update.]
Another simple, but I hope useful, color toning automation script designed to assist in adjusting flesh tones in portraits. It doesn't do anything that you couldn't do on your own a few other ways, but if you like fiddling with the complexion of subjects in portraits, and like me, you are a tad lazy, you might find it helpful.
By adding or subtracting small amounts of magenta and yellow you can change someone's skin tone to more closely resemble their natural coloring in your image. Then by either adding or reducing the cyan from the skin tone, you can give your subject more of a "suntan" or, conversely, look more fair-skinned.
This filter creates three new layers, one for each color, to set up an environment where you can more easily tweek your model's coloring. After running the filter, you can then make minute changes to the opacity of each layer in the layers dialog box, as well as change the mode: ideally to either addition or subtraction, but perhaps other modes could give good results, too.
For simplicity this script uses the red channel to create the layer masks. If you find the colors bleed into unwanted areas, more experienced uses may wish to edit the layer masks by painting on them with either a black or a white brush to more precisely control how the added layers affect the rest of the image.
In the menu it can be found at Colors -> CMY Tone Adjustment