Cocoa Mulch (do not use in flower beds that your dog has access to)

Chocolate (6 oz of chocolate can kill a 24 pound dog.)

Apple seeds

Avacado (including guacamole)

Walnuts and Macademia nuts

Grapes and raisins

Onions

Large quantities of Garlic

Sugar free foods containing Xylitol
(reference: http://www.msnbc.msn.com/id/15066760/)

Antifreeze - even a very small amount can lead to kidney failure and death. Be on the outlook when walking through parking lots! Dogs will lick antifreeze because it has a sweet taste.