This is a simple python script, which creates spirograph type curves. There are endless varieties which can be generated. There is a page on my site with some documentation:
This is a simple script to invert the brightness of only the dark sections of a picture, without changing the colour. You can change the threshold for the script, so that it creates a brighter picture, or one that isn't as bright. The effect is applied on a separate layer above the active one, so you can delete it or apply a layer mask to it if you want.
Automaticly copies the image and insert this. The script creates a Layer-Mask and makes a part of the insert layer invisible.
It's great to have a part of the original image on the top, because than you have a very well comparision.
If you have any suggestions for improvement contact me.
I hope this example conform your requests:

The script adds an "Extend to Image Size" command under the Layer Menu which will scale the (visible portion) of the active layer so that it has the same dimensions as the image canvas. read more »
Windows binary for this new plugin by Arel Welgan.
Source has been fixed for Windows compilation by Samj. Plugin no longer crashes under Windows.
Please note that all comments below, prior to Nov 28 2009 are related to another build not to this
that seems working well with RGB and RGBA images
Extract colour-match.exe from the ZIP archive and copy it to your c:\Documents and Settings\[user name]\.gimp-2.6\plug-ins\
(As Windows compilation by Samj was perfect, I kept it in the archive. Mine was several bytes larger...)
Easily map colours from a source colour to a target colour. Very effective for matching skin tones across images or between people.
Relatively quick (but not earth shattering). It doesn't work with grayscale or indexed images. Has preview, works with selections and layers, etc. Very, easy to use, and quite useful for portraits or recolouring graphics.

A demonstration: Lenna's face is mapped to the colour of the feather on her hat. This was done by selecting a colour from her face, then selecting a colour from the feather. read more »
This script generates two difference layers from the two layers on the top, A is the original image layer, B the modified (e.g. sharpened) one on top:
A - B = C
B - A = D
A - C + D = B read more »
This script inverse the visibility of the active layer. It is very useful because you can give it a short cut and use the mouse only on your image!
This script cycles through layers making the active layer the only visible layer (useful for hand-editing simple animations) similar to Layer > Stack > Select Next/Previous Layer.
Scale and move the layer to cover the canvas. Just another "shortcut" script for a commonly used action...
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