20 of the World's Deadliest Plants

Hippomane mancinella (aka Manchineel, manzanilla de la muerte ["little apple of death"])
The Manchineel tree does not like company. If you brush up against the tree, the white sap it produces is liable to produce a violent allergic reaction on your skin. Standing beneath the tree during a rainstorm will cause any skin that comes in contact with runoff from the tree to blister.