This script changes a dry landscape to a wet one. I scripted my tutorial from here:
http://www.flickr.com/groups/gimpusers/discuss/72157623477454455/

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.

Before:
http://www.flickr.com/photos/kenlund/3378248128/


After:
http://www.flickr.com/photos/28653536@N07/4426160982/


Update1: Some more options.