Avocadoes - Avocadoes contain persin, a fatty-acid like substance that is toxic to many species of animals, and may cause vomiting and diarrhea in dogs.
Bones - Chicken, fish, pork or beef bones can cause choking or can splinter when chewed, creating sharp edges that can cut a dog's mouth, esophagus, stomach or intestines. If swallowed, bones can cause an obstruction of the intestines.
Chocolate - Theobromine, a stimulant found in chocolate, affects the central nervous system and causes increased heart and breathing rates, irregular heart beat, restlessness, hyperactivity, vomiting and diarrhea. High doses can lead to seizures, coma and death. Bitter chocolate, such as baking chocolate, contains much more theobromine than milk chocolate. For milk chocolate, a toxic dose is 1 ounce per pound of body weight, while for baking chocolate, a toxic dose is only 0.1 ounce per pound of body weight.
Corn on the cob - If a dog bites off and swallows a piece of corn cob, it can block the intestine and must be surgically removed.
Grapes and raisins - While the toxin contained in grapes and raisins has not yet been identified, it is known to damage the kidneys and cause kidney failure in dogs if ingested in large enough amounts. The estimated toxic dose is 1/3 ounce of grapes or 0.05 ounce of raisins per pound of body weight. The symptoms are lack of appetite, lethargy, abdominal pain, vomiting and diarrhea.
Macadamia nuts - Both raw and roasted macadamia nuts contain an unidentified toxin that can cause elevated temperature, weakness, loss of muscle control, tremors, stiff joints and paralysis of the hind legs. the toxic dose ranges from 2.4 to 62.4 grams per pound of body weight. This wide range indicates that some dogs are much more sensitive to the toxin than others, and can be affected by eating only a small amount of nuts. The toxicity is increased if the nuts are eaten in combination with chocolate.
Milk - Although tolerance level varies, most dogs do not have enough lactase enzyme to digest the lactose in milk, and milk will cause digestive problems and diarrhea.
Onions and garlic - Onions and garlic contain thiosulfate, which can damage red blood cells and can lead to hemolytic anemia. Symptoms include weakness, lethargy, vomiting, diarrhea, blood in the urine, and breathlessness.
Raw eggs - Raw eggs contain avidin, which can prevent absorption of biotin, an essential B vitamin. A biotin deficiency can lead to problems with skin and coat. Raw eggs may also contain Salmonella bacteria.
Yeast dough - Unbaked bread dough made with yeast can expand in a dog's stomach, distending it and causing severe pain.
Xylitol - Xylitol, a sugar substitute found in sugar-free gum, candy, and products for diabetics, is extremely toxic to dogs. Even a small amount, such as 2-3 sticks of sugar-free gum, can cause a drop in blood sugar level. Symptoms include lethargy, vomiting, difficulty standing or walking, and if left untreated, can result in fatal liver failure.
Sources:
http://www.peteducation.com/article.cfm?c=2+1659&aid=1030
http://www.petcaretakers.com/dog-nutrition/do-not-feed.php
http://www.aspcabehavior.org/articles/71/Foods-That-Are-Hazardous-to-Dogs-.aspx