Ricin

Ricin's a protein from castor beans - yep, the same ones used to prepare castor oil. The parent plant is sometimes used as a warm-weather ornamental, and used to be an industrial crop. The leaves aren't a problem, but the beans contain up to 5% ricin, which is a rather high yield for a natural product. It's quite toxic, although there are certainly worse things out there. Botulinum toxin, for example, is a thousand times more potent, but you can't grow anerobic bacteria very well in your back yard.
· Castor beans [Dlowe via InstaPundit]