Yerba mate
An August 11, 2005 United States patent application (documents #20050176777, #20030185908, and #20020054926) cites yerba mate extract as an inhibitor of MAO activity; the maximal inhibition observed in vitro was 40–50%. A monoamine oxidase inhibitor is a type of antidepressant, so there is some data to suggest that yerba mate has a calming effect in this regard.