This is a script which simulates the typical film grain of high ISO pictures. I scripted exactly this description:
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Generic -> Film Grain.
Holdness and Value are two options directly taken from HSV-Noise, Strength changes the amount and area of grey, which will turn grainy.
It works together with colored and greyscaled pictures.
Update1: Two new options: Grain Blur and Desaturate Image.
Update2: B/W as extra layer.
For all scripts together, go here: