This script will make a grid of circles. You input the diameter of the circles, the number of circles in the X and Y directions, the space between the edges of the circles, and if you want the circles in a rectangular or a hexagonal grid. The two attached images show pictures of the default rectangular and hexagonal grid created by this script. The circles and background can be any colors. This script could be useful for selection masks, making pictures of bubble wrap, or lots of other things that I haven't thought of!
The script shows up in the Filters/SambrookJM menu in GIMP, but this can be changed by editing the last line in the .scm file. Copy this into your GIMP scripts directory, and it should work just fine. If anyone has any problems or suggestions, let me know and I'll try to fix it.
Update #3 - Now with circle, square, hexagonal and octagonal goodness! I got rid of the earlier versions of the script, so the one posted here is the correct one. Diamonds will probably be a different script, since there are several other variables that are specific to diamonds that aren't used by the other shapes.