GradFilter simulates a graduated neutral density filter. GradFilter Pro extends support to allow multiple graduated filters of any color, on any layer. Either plugin can be used, for example, to darken a bright sky or brighten a dark subject so that both the sky and subject are properly exposed. In an ideal world, a real physical filter would be used as this maximises the dynamic range of the image hitting the film plane/sensor, but this plugin is still quite useful in many situations where one doesn't have such a filter to hand, especially when working with RAW files. In some ways, the plugin provides a greater amount of flexibility, by stretching the physical constraints of a real filter to allow the "addition" of light.

GradFilter is available for free and includes a functional set of features for applying a graduated neutral density filter to the main layer.

Hi! Here you can find my plugins written for Bibble, a RAW developer and professional workflow management software by BibbleLabs.
For me the main advantages of Bibble are its portability between different operating systems (it's the best RAW developer for Linux) and its extensibility by third party plugins, which will help you to perform typical photo-editing tasks without the need to open other editors like Photoshop or Gimp.
This page contains a good collection of plugins developed by others for Bibble.