The rule of thirds is a compositional rule of thumb in visual arts such as painting, photography and design. The rule states that an image should be imagined as divided into nine equal parts by two equally-spaced horizontal lines and two equally-spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections. Proponents of the technique claim that aligning a subject with these points creates more tension, energy and interest in the composition than simply centering the subject would. - Rule of Thirds at Wiki
This is what the imaginary grid looks like, and how you may use it to place your subject somewhere besides the center of the image:
From Wikipedia
So I've searched for examples of pictures using this rule, and I've found some superb examples of usage. These are amazing, and you should really visit each one of these photographers (by clicking the pictures) for more great pictures, and even buying some of these images. I'll probably buy one or two to hang in my wall. I hope you enjoy these. Cheers! ;)