The Degrain plugin is licensed under the GPL.
The source code is available here.
gimptool --install degrain.c will compile the plugin and install it in your local Gimp plugins directory.
The plugin appears in the Filters/Enhance menu as Film Grain Remover....
The degrain code was originally written by Stefan M Fendt, who asked me for some help in turning it into a Gimp plugin. The three sliders allow you to adjust the effect on the red, green, and blue channels separately.