Scorpion venom

Fact: The most venomous scorpion in the world is the Deathstalker scorpion, which resides mostly in the Middle East and North Africa. Its venom is a powerful neurotoxin which can kill, especially the young or the weak, but the venom is also under consideration as a possible treatment for cancer.

Quote: There is poison in the fang of the serpent, in the mouth of the fly and in the sting of a scorpion; but the wicked man is saturated with it. -- Chanakya

And a site: A NOVA article about the healing powers of different types of venom.