A recent purchase of mine is this wonderful steel Phi Ruler. It helps me measure out "Golden Rectangles". A Golden Rectangle is one whose long side and short side are proportional to Phi (the Greek letter φ). That is, one side is 1.618 times longer or shorter than the other. This creates a shape that has a strong visual appeal, especially in architecture and graphic design but also in other visual arts. I believe the reason for this might be physiological in humans, and the idea has been used since ancient times.
Above is an example of a Golden Rectangle. Very roughly, you can make one 5 x 8 inches. You can fit two of these on a standard sheet of paper if you are making handbills. A Phi Ruler makes measuring these dimensions a snap, as I have yet to find a visual utility for Photoshop that will do this.
This is an excerpt from the instructions page on how to read the Phi Ruler. I don't know who manufactures these things, so I cannot at this time give a proper credit.