This script adds a realistic film grain effect to a black and white or toned image. To install, copy the attached file to GIMP/share/gimp/2.0/scripts. The larger the file you are running it on, the larger the value you should have for Strength. The value for Sharpness depends on your preferences, but I usually start at 1 and go from there. The lower the value, the sharper the noise.
Plugin to load Photoshop PAT patterns set as image in GIMP. Multiple patterns included in a file are loaded as layers. Should handle Grayscale, Indexed and RGB patterns ; anyway, a RGB image is always created for now. Requires Gimp 2.4.