Contrary to what their name might suggest, mangosteens are not related to mangoes. Primarily grown in Southeast Asia, this fist-size fruit has a thick purple rind and a soft interior made up of white segments. The rind (or pericarp) contains compounds called xanthones, which may combat cancer and inflammation. Though the fruit itself is difficult to find in the continental United States, you can purchase mangosteen-juice products that incorporate xanthones from the rind.
Available at health-food stores and online; try vitacost.com.