Adobe Photoshop Lightroom has a nice Crop Overlay feature, which allows you to view your images with different grids, or overlays, on top of your images. There are six (6) different crop overlays available. Here is what you do:
1) Hit the 'R' key to go to the Develop module and have the Crop feature enabled.
2) The default display will be the 'Rule of Thirds' crop overlay.
3) Cycle through the other overlay options by clicking the letter 'O' on your keyboard. Not the zero key. For those overlays that are not symmetrical, you can use the Shift+O combination to alter the orienation of the overlay.
I have been playing around with these different overlays over the past few days, and I am most intrigued by the Golden Ration and Golden Sprial overlays.
Rule of Thirds
The above image and below image are known as the 'golden mean' or 'Phi'. They occur frequently in nature, and some studies have shown that humans may be genetically programmed to recognize the ratio as a pleasing ratio or composition. Studies of top fashion models revealed that their faces have an abundance of the 1.618 ratio.