The one that immediately comes to mind is the use of Yaupon Holly (Ilex vomitoria) by native Americans. The story is that they used to use it to get hyped up for battle and to purge (hence "vomitoria"). This is probably more misrepresented history than folk lore, but there's a fine line between those things.
PS: Yaupon Holly is one of the only (maybe the only?) native plant in the US that has caffeine. Its cousin in South America is actually the plant they use to make yerba mate.