Snake

I wrote about the origin of coumadin some time ago, and now I've discovered another drug developed from a quite lethal naturally occurring substance. The venom of the Bothrops jararaca Brazilian pit viper contains a peptide that blocks angiotensin converting enzyme. While the venom is orally inactive, this finding lead to the development of the incredibly common antihypertensive class ACE inhibitors.

Image of Bothrops jararaca snake shown under GNU Free Documentation License, from Wikipedia.