This script-fu rotates an image -/+ 26.57 degrees to help to make an isometric roof. There are some little options to resize, to cut the semi-transparent pixels, to merge down.
I don't know if this script will be very useful. It is for me, so I am sharing.
You can find "03 - Isometric Roof (26.57 degrees)" in Filters>2D Isometric-Fu>