I don't remember where I heard it but one of the 'rules' is that the smaller the creature, the more toxic the venom. Jelly Fish, Frogs, spiders, Octopii, etc. There are exceptions but for the most part is holds true.

Speaking of Octopii I'm surprised nobody mentioned the Blue Ringed Octopus. One of natures most venomous creatures
http://en.wikipedia.org/wiki/Blue-ringed_octopus