Mila Kunis, is not alone when it comes to Heterochromia. Here's just some of the celebrities with the condition:
Jane Seymour ("Dr. Quinn, Medicine Woman")
Superman Henry Cavill,
Elizabeth Berkeley ("Saved By the Bell" and "Showgirls")
According to Wikipedia, Heterochromia is a difference in coloration, usually of the iris but also of hair or skin. Today, Heterochromia is celebrated because it's National Different Color Eye Day.