GIMP Plugin Registry

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.
Another examples can be found here and here:

It works together with colored and greyscaled pictures.