THC from the cannabis plant. It is a very long list though, plants go to a great deal of effort to deter pests so the list would be more limited by the subset of plants that humans find useful to cultivate.
side note: It kills you by making all your muscles tense so strongly that you can't breath any more. The muscles in your face tense in a way that it gives you whats called a "Strychnine Smile".