Vignette is that effect you sometimes see where the corners of a photo are a little darker than the rest of the photo. Here's an example from Aaron Escobar's photostream on Flickr (click on the photo to go to the Flickr page):
Here are the steps to create this effect in Gimp:
* Go to the "Layer" menu and choose "New Layer" -make sure the default layer type of "Transparency" is selected, then select "Ok".
* In the toolbox, click on the Ellipse Select Tool and set the Feather edges to "100" (push the slider all the way to the right).
* Select the area where you want the vignette effect to end.
* When the area is selected, go to the "Select" menu and click "Invert" -this will select everything OUTSIDE of the circle instead of the circle itself.
* Click on the Bucket Fill Tool from the Toolbox. Make sure black (or whatever color you want your vignette to be) is selected, then click in the selected area to fill in the color.
* Adjust the layer's opacity to make the vignette go from black to more "see through" -by changing the opacity you can make it as dark or light as you choose. (The "Layer" window can be opened by clicking on the "Windows" menu, then going to "Dockable Dialogs" and selecting "Layers".
* Chances are the transition between the vignette and your photo is a bit more abrupt than you'd like it to be. That's ok we can make it more gradual...
* Click on the "Filters" menu, go to "Blur" > "Gaussian Blur".
* Move the blur radius up from the default "5" to somewhere around "150" depending on the transition you are looking for.
* When it looks good, flatten the image and save it and you're done!