GIMP Plugin Registry

This extension allows you to add dominants to your black and white (or colour) photos.
In the "Layer mask" mode the dominant is balanced with a mask, such that your whites remains whites.
The homepage is in Italian but you can see some screenshots and examples there. Please note also that the plugin is in English.

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After

FX-Foundry/Photo/Enhancement/Maximize Local Contrast...
Like Stretch Contrast, but instead of using the minimum and maximum pixel values from the entire image, uses the minimum and maximum in the vicinity of a pixel to compute its new value. Takes a while to run, but really brings out textures. Result is a Grain Merge layer that adjusts values of the original image.

This filter operates on the visible image, not on a layer or selection.  read more »

gimp-netpbm2gimp-plugins is a collection of over 200 GIMP plug-ins produced by running the source code of the Netpbm graphics filters through Netpbm2Gimp, which is described below. (Note: Think "GIMP" where the Netpbm graphics filter documentation says "PBM", "PGM", "PPM", "PNM", or "PAM".) There is a particular emphasis on load/save plug-ins for obscure graphics formats, but a number of miscellaneous and useful image filters and image generators are included as well.  read more »

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.

This is another script for converting a picture to black & white. The decompose plugin must to be available. The way of converting the picture is rather different than what other plugins do. Two different luminance layers are mixed according to a mask made with a saturation layer. Six different combinations are provided. Functions are available under the menu script-fu/Photography/BW Labs.

A set of gradients for toning B&W photos. New are multi-colored toning gradients (thanks Ron Antonishen for the conversion script), and 3 Albubum print gradients. Instructions for use are in the README file.

Note: you will generally get better results from:

->color->map->sample colorize

than from

->color->map->gradient map