The Polar Monitor Bitmap is a simple text-based 1-bit image file format. It is used for uploading logos to Polar watches (http://www.polar.fi/) using e.g. the Polar UpLink Tool or the Polar ProTrainer software.
Build and install:
$ gimptool-2.0 --install file-pmb.c
When saving images in this format, you will be able to specify a quality factor that can range from 0 to 100. The higher the number, the larger the file produced and the better the quality of the image.
This Python plugin simply can lay out all layers of an image into a grid (with the grid settings changed to reflect the old image size). And it also does the inverse, creating one layer for each grid cell in the image. This can be useful for things like a sprite sheet or glyph sheet and I didn't find anything similar.