GIMP Plugin Registry

Quantizes (reduces color count) and traces outlines or centerlines, into a new image. Optionally creates paths for the outlines or centerlines. Also can be used to generate an SVG file for an image.

Calls autotrace (a separate program that must also be installed) to do the tracing (to SVG, a vector file format) and then imports the SVG back into Gimp.

This plug-in is designed for batch watermarking and resizing of image files. It lets you choose the directory which contains the images, the file extension, the font, the copyright text and the options associated with the processing. Useful for photographers who upload a lot of photos on the web and don't want to watermark every single file individually. The resulting images contain between one and four instances of the copyright text (selectable as an option).

I just notice that MathMap one of the better Gimp Plugin is not listed here.
To present it i just quote its developer Mark Probst
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MathMap is an image processing application that works on several levels:

* Without any prerequisite knowledge about image processing, one can use any of the more than one hundred filters that come bundled with MathMap.

This plugin creates a burnt-paper effect. It uses your selection as the original outline of the paper. It then creates an old looking piece of paper that is burnt around the edges and overlays the current layer atop this paper.

Here is an example of its output:

That image was created from a text layer and a selection that was larger than the text area:

Here is the plugin source file