The ribs and side torso area is a rather common location for a tattoo design, perhaps more common among women, and the tattoo often extends below the ribcage in many cases.
The type of designs range from anything under the sun, or anything you would have inked elsewhere on the body, however flowers and other vegetation seem to be most popular.
Feast your pretty peepers on this picture gallery of excellent rib tattoo ideas, which are mainly for girls, however guys may find them appealing as well.