Well, just about every animal is poisonous or venomous in some regard. Your saliva is actually toxic to anything you bite, though the toxins are too weak to cause death.
Cephalopods have a very well evolved system of highly toxic salivas that can be injected or masticated into a bite area, some so strong (like the blue ring) that they cause almost instant paralysis in prey items, or anything attacking the animal.
A lot of octopus species are venomous to a high degree, new research is finding more and more about just how toxic.
The smaller species tend to be the most venomous, many of them with warning colours, like the blue ring. Some just cause temporary numbness, others cause shooting pain and muscle spasms. Yep, you have to be careful with all of them !
i saw a documentary that said that the Metasepia flambuoyant cuttlefish are venomous to humans. another one that comes to mind is the pajama squid, Sepioloidea lineolata. Octopus mototi, which looks like a bimac, is also poisonous
There's a discussion around about how vague the details were in that documentary, and how it appears that Mark Norman hasn't published those results. It's worth mentioning that there are many cephalopods that have never bitten a human, and probably many that have never been eaten by a human, so caution is wise. Roy has also pointed out that even a non-envenomating octopus bite breaks the skin and exposes your body to tank or sea water, which harbors all sorts of weird bacteria and such, and even "non-venomous" octos have venom that people react to roughly like a bee sting, which probably means that, as with bees, some people are deathly allergic to octopus bites even if most people are fine.
That all being said, blue ringed octopuses are distinctive in being very, very, very toxic. Unlike regular cephalotoxin, the TTX produced by symbiotic vibrio bacteria, which is the same toxin exploited by pufferfish and some newts (or is it salamanders?) is used by neurobiologists because it's so effective, even in tiny amounts, and shutting down a vital part of nerves. While this is useful if you want to study how nerves work when part of them is broken, it means that very small amounts of octopus venom injected into the bloodstream will cause rapid respiratory paralysis. (It's a good thing that it doesn't cross the blood-brain barrier, since it would shut down the brain, too) and while the heart and brain stay intact, you can't move and you can't breathe. The good news is that this is reversible, so if you're put on artificial respiration for a few days, it will wear off, but if something outside your body (CPR, ventilator) isn't breathing for you, you'll get no oxygen and die. It takes very little venom to cause this, so the bite of a small blue-ring is not just life-threateningly dangerous like a rattlesnake or a black widow, it's really, really likely to be lethal, and pretty much depends more on whether the venom gets injected into the bloodstream than whether your body can deal with it. If blue rings are like bimacs, they have some control over whether they inject venom with their bites, so not all blue-ring bites are deadly, but if you are envenomated, you are in deep trouble.
In that documentary, Norman says that the flesh of Metasepia is "more toxic" than blue rings, but it mentions that they looked for it in the saliva and other places, and found it in the flesh, which might mean that it's safe to be bitten by one and unsafe to eat one. But until he publishes the actual findings, I think Metasepia should be assumed to be lethal. No one, to my knowledge, knows whether mimics and wunderpuses are toxic in any way (many bright-colored animals are, though.) The reports from Norman on the "pajama squid" are even more vague, and I hadn't heard about O. Mototi before (although google shows that Roy Caldwell mentioned it on TONMO in 2002 as "natives report that this octopus is poisonous" or similar). Another octopus, Eledone cirrhosa, has a toxin that is useful for some medical purpose I've forgotten, but I don't know if its bite injects enough that it causes problems for humans... most cephalopod salivary venom is most effective against crustaceans, since it's used for prey subdual.
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