GIMP Plugin Registry

Creates a new layer with a grid, rotated 45 degrees, to help you choose corners for cropping.

Move the grid until a line intersects a feature in your image, then crop so that the grid line meets a corner of your cropping rectangle. You can then use the grid to choose the opposite corner, but make sure you don't change either of the sides of your original corner.

Installs in FX-Foundry/Toolbox/Grids/Diagonal Grid