There are many, many different types of cleavage, but the one that seems to cause the most controversy is the underboob. No wonder Rihanna (seen above, performing this weekend) is on board -- she's such a rebel, that girl! Is a glimpse of a lady boob's bottom half sexy or silly? Sultry or stupid? Does it make you wonder, I wonder what the rest is like? or Hey, what happened to the rest of that chick's shirt? Undecided? Click through these 15 great moments in celebrity underboobage and then tell us how you feel.