List of Poisonous Household/Garden items

Foods

Apple seeds
Rhubarb leaves
Garlic (in excess)
Chocolate
Avocado
Onion
Kale
Raisins
Grapes
Green tomatos (tomatine) and plants
Walnuts
Macadamia nuts
Apricot kernels
Cherry kernels
Onions
Artificial sweeteners
Caffeine (coffee, tea bags, diet pills, etc..)
Alcohol


Medications
It is not advisable to give dogs human medication, regardless of toxicity, unless under recommendation from your vet

Paracetamol
Ibuprofen
Advil
Oral contraceptive pills


Other household products

Bleach
Rat poisons
Ant killers
Batteries
Antifreeze
Brake fluid
Slug bait


Plants

There are many poisonous plants, so it's always recommended to familiarise yourself with them through research. For a full list of poisonous substances, I would recommend a website such as: http://www.petpoisonhelpline.com/poisons/

You can put Brake fluid on there as well. It's also (supposedly) sweet tasting and dogs love it.
A friend of my previous landlord lost all 3 his dogs to it. They were working on their car and didn't know the dogs would lick the fluid so just put the pan on the floor next to the car. They rang me in the middle of the night for a Vet number and had to race to try and save the dogs. Unfortunately it was to late and a very sad lesson to learn.

Thanks - yes, it contains the same toxic (and good-tasting) ingredients as anti-freeze. I'll add it to the list.

I was just about to say anti freeze.

But walnuts? Nooooo, that is simply not true. (Allergies excluded) Jess should be dead then.

Walnuts are what I trained Jess with, she has eaten them all her life & sometimes in large quantities if she stole the container.  
They are candy to her. I spoke to our vet as she even ate the shells once. I was told, the actual nut, non toxic to dogs but the husk is toxic, should be ok as she was an mature dog but watch. It maybe uncomfortable coming out as the digestive fluids will not break down the shells. Hazel nuts are also one of her favorites.  A family GSD used to eat them, we gave him the nut, he'd crack it, spit the shell out & eat the nut. Never a problem.

Tyson has walnuts if he is a good boy. Never a problem with him either. Not even an upset tummy when he gulps them down with me.

Jess is quite fond of a pecan or a pistachio or 2. ( 2=500.)

Walnuts would need to be consumed in large quantities, but they can cause problems if the toxins from the husks remain on the nut. Not very common though.

Like you said, the husks do not break down in the digestive system, so they can cause blockages in smaller dogs. Ouch!

One of the biggest risks of walnuts is the toxins, from fungus/mould, that grow on them when they come into contact with damp, even if it's a small amount. Generally speaking, that's why most people avoid feeding them to dogs.

Ahhhh, now the mould is quite another subject to the flesh.
I thought you were referring to the nut meat itself.

At the time I typed up the list, I just assumed that walnuts themselves were dangerous. I only really looked into it a while ago, out of curiosity.