Vignetting means the darkening of image corners in a photographic image. It is usually accidental and occurs due to the improper use of attachment to the camera lens. It is a common occurrence and photographers usually use Photoshop to reduce this effect.
The image below taken from Wikipedia shows the vegnetted image.
Vignetting Reducer is a free tool for Photographers who need to reduce the vignetted image effect from their photos. Users just need to open the image that they want to fix, and load the mask, make some adjustments, crop the image(optional), and hit Save Image to save the new image. The original photo can be seen on the left side while the output image can be seen on the right.
It has a build-in settings from where you can check and select the hotspot. It is not a professional photography tool but can be useful for quick filtering.
It works on all versions of Windows. Enjoy!