There's a lot of common foods (or things that might be mistaken for food) that will make a dog sick, or even kill em. Here's a list of some of the household items a dog might be likely to eat that e shouldn't. Many common yard plants are also poisonous--if you are worried about them you might want to read more here and here.
Chocolate: baking chocolate is the worst, needing only 0.1 ounce per pound of body weight to kill a dog. Cocoa and milk chocolate should also be avoided. White chocolate is the least toxic, requiring 200 ounces per pound of body weight to cause death. It's the theobromine in chocolate that kills, found in chocolate liquor, coffee and tea.
Bones: Most types of bones will splinter, and these splinters may become lodged in the dog's throat. The best bone for a dog is the beef shin bone. Avoid chicken and pork bones.
Cassava root
Rhubarb
Onions: especially in large quantities. Onions are even more dangerous to cats.
Poisonous Plant Parts:
Apple Seeds
Apricot Pits
Peach Pits
Pear Seeds
Avocado Leaves
Eggplant leaves
All parts tomato plant except the fruit
Uncooked Java beans
Acorns
Tobacco, Marijuana, and Aspirin are also bad for your dog.
Alcohol isn't poisonous, but dogs will get drunk much more quickly than a human.
Caffeine: I haven't had much luck trying to find out how poisonous caffeine is to dogs, but I wouldn't advise giving them tea, coffee, jolt, etc.
Rich, fatty foods can cause pancreatitis.
Dairy products:Dogs are lactose intolerant. Dairy products can cause excessive gas and diarrhea.
I hope I don't have to tell you this, but Aluminum Foil can cut a dog's intestines, causing internal bleeding, and Plastic food wrap can cause choking or intestinal obstruction.
Rawhides, cow hooves, and pigs' ears are hard to digest, and may cause vomiting or diarrhea if eaten too quickly. Cow hooves are hard enough that they can break a dog's tooth, and sharp splinters can become lodged in the intestinal tract.
Finally, watch out for Bloat (gastric dilatation and volvulus syndrome or GDV), a condition that comes from eating too much too fast, especially in times of stress or just after exercise. Changes in diet and gas producing foods may also contribute to this condition. Most often occurring in large dogs, the symptoms are; a distended abdomen, abdominal discomfort, severe weakness and shock. This condition can be life threatening, and needs to be treated by a veterinarian as quickly as possible.
http://www.dog.com/vet/holidays/04.html
http://www.inch.com/~dogs/safetytips.html
http://www.geocities.com/silverhawkaussies/poison.html
http://www.monmouth.com/~spca/poison.html
http://www.canineplanet.co.uk/hart004.htm (chocolate)
http://www.comportone.com/cpo/animal/articles/chocolate.htm (goes into more detail on chocolate).
http://cardogz.com/columns/vet/archive/2000_10_18.shtml (goes into even more detail on chocolate).