Since a very long time, my TODO list included a complete review of digiKam Film Grain tool. Based on an old Gimp Guru tutorial, this tool lacks a lots of features as the capability to manage colors and luminosity noise, graininess size, and photo-electronic noise rendering.
I have always been impressionned by PowerRetouche photoshop plugin. This company make great tools for photographers, but of course, all are closed source and non-free.
This is a companion discussion topic for the original entry at https://digikam.org/news/2010-03-26_digikam_film_grain_tool_re-written/