Apparently, zombies are not just for movies - the story of Clairvius Narcisse of Haiti is the only real-life documented case of someone being made into a zombie by use of poisons:
The story begins in 1962, in Haiti. A man called Clairvius Narcisse was sold to a zombie master by his brothers, because Clairvius refused to sell his share of the family land. Soon after Clairvius "officially" died, and was buried. However, he had been later secretly unburied, and was actually working as a zombie slave on a sugar plantation with many other zombies. In 1964, his zombie master died, and he wandered across the island in a psychotic daze for the next 16 years. The drugs that made him psychotic were gradually wearing off. In 1980, he accidentally stumbled across his long-lost sister in a market place, and recognized her. She didn't recognise him, but he identified himself to her by telling her early childhood experiences that only he could possibly know.
Links: Zombie, The Zombie Poison (via Cynical-C) - Thanks Myra!