Photoshop has a snap to grid feature you know (just photoshop costs and arm and a leg and a kidney), also the grids usually are not part of a picture, they are just on the UI. Snap to grid is really useful when you are actually drawing something yourself rather than just editing an existing image.
Does this also have the ability to snap to grid points? If not could it be added? Edit: Noticed a post above, just to clarify, I need to draw something not position it, so I need to be able to snap to a grid point, draw a line, snap to another grid point, etc. So the PseudoSnap plugin mentioned would not help there.