According to Wade Davis (an ethnobotanist who was working at Harvard at the time), in his book The Serpent and the Rainbow, zombies are made by vodoun witch doctors (bokors)* using a mixture of plant and animal matter.

During his lengthy visit to Haiti, Davis was able to characterize the basic procedure as follows. After the potential victim is identified, crushed glass mixed with a toxic powder is placed by the voodoun priest in a location where the individual will walk. Walking upon this trap cuts open the feet, and results in the toxin being introduced into the blood stream. After the victim is thus poisoned, he/she begins to exhibit a number of symptoms including total paralysis, a cooling of the body temperature, highly reduced respiration rate and very serious delusions. The victim is often taken for dead by his neighbours and family, and is summarily buried. Shortly after everyone leaves, the vodoun priest then unearths the body, and convinces the victim that he/she is now a zombie and is under the strict control of the priest.

As far as Davis was able to determine, this procedure does in fact take place, albeit rarely. The toxic mixture seemed to be, upon scientific analysis, mostly herbs and spices, but there was evidence of the use of a marine worm (polychaete) which contains tetrodotoxin, a powerful nerve poison. Davis believed that the reason why vodoun bokors were able to convince living people they had become zombies was due to the powerful effects of the tetrodotoxin combined with the powerful social belief in the magical power of the witch doctors.

* This is, if I remember correctly, the correct way to spell and refer to the religion widely called voodoo. Also, props go to Cletus the Foetus for correcting my errors in an earlier revision of this w/u.