Vignette Corrector compensates or apply vignetting effects
Edit by : Vignetting is the brighness fallof at the corners of an image. This is a rather universal phenomenon, however it is more pronounced for wide-angle lenses. If you are interested in the theoretical aspects of vignetting, here is a nice explanation.

Vignetting can be very disturbing if you want to combine a set of images to a Panorama (panorama stitching). You want to compensate that unwanted effect on the individual frames before stitching. On the other hand, vignetting is sometimes deliberately used as a nostalgic style element in portray photography.
Vignette Corrector offers you a number of controls to adjust the size and brightness of the vignette. In addition you can modify the tint and saturation. I found this useful for scanned negatives, where I could not compensate vignetting with brightness modifications alone. I have also implemented a 3x3 mirror Tiled view to enhance the vignette visibility. Use the tiled view for accurate vignette correction prior to panorama stitching.

Below are screenshots of the Vignette Corrector plugin. Click on the image to display the next one in the cycle you can free download Vignette Corrector 5.2 now.