This script changes a dry landscape to a wet one. I scripted my tutorial from here:
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Light and Shadow -> Rainy Landscape.
It doesn't have many options, because you can change them afterwards in the layers menu (ctrl-L).
If you like it, hug someone.
An example can be found here:
Update1: Some more options.
For all scripts together, go here: