Chocolate:
The ingredient in chocolate this is harmful to dogs is called theobromine. It's a stimulant that comes from the coca plant. The concern comes from the dogs heart. Like too much of any stimulant, the theobromine can cause the heartrate to speed up too much. More dangerous in older dogs. People who race horses use theobromine to get more from the horse when racing them.
If you can find chocolate that has no theobromine in it, it's perfectly dog safe and your dog will love you for it. As to where you might find this, I haven't a clue. According to Hershey's: "Currently there are no theobromine-free chocolate products available to consumers."
If you're a gardner, you might want to avoid coco mulch as well. Hershey's claims that it WILL harm dogs (and did so in 50% of tested dogs) but they also claimed that 98% of dogs won't eat it.
Grapes:
"During the twelve-month period in which the effects of grapes were studied, the ASPCA Animal Poison Control Center handled 140 cases involving one or more dogs. Over a third of the dogs developed symptoms ranging from vomiting to kidney failure, and seven dogs died. The ASPCA based their study on reported cases, so naturally there may be cases where a dogs health is entirely unaffected by eating grapes. But until they know all the facts, the Society advises against feeding pets grapes or raisins in any amount."
Nuts:
"Macadamia nuts should be avoided. In fact most nuts are not good for a dogs health since their high phosporus content is said to lead to bladder stones."
Onions:
"Onions, especially raw onions, have been shown to trigger hemolytic anemia in dogs. (Stephen J Ettinger, D.V.M and Edward C. Fieldman, D.V.M. 's book: Textbook of Veterinary Internal Medicine vol. 2 pg 1884.) Stay away from onion powder too."
Onions and garlic contain the toxic ingredient thiosulphate. Onions are more of a danger.
Potatoes:
"Potato poisonings among people and dogs are rare but have occurred. The toxin, solanine, is poorly absorbed and is only found in green sprouts (these occur in tubers exposed to sunlight) and green potato skins. This explains why incidents seldom occur. Note that cooked, mashed potatoes are fine for a dogs health, actually quite nutritious and digestible." Mostly potato peelings and green looking potatoes.
Artificial Sweetners:
"Xylitol is used as a sweetener in many products, especially sugarless gum and candies. Ingesting large amounts of products sweetened with xylitol may cause a sudden drop in blood sugar in dogs, resulting depression, loss of coordination, and seizures. According to Dr. Eric K. Dunayer, a consulting veterinarian in clinical toxicology for the poison control center, "These signs can develop quite rapidly, at times less than 30 minutes after ingestion of the product" states Dr. Dunayer, "…therefore, it is important that pet owners seek veterinary treatment immediately.""
Turkey Skin:
"Turkey skin is currently thought to cause acute pancreatis in dogs, partly due to it's high fat content."
Other foods listed by the ASPCA as harmful:
Alcoholic beverages
Avocado (the only "fatty" member of the vegetable family)
Coffee (all forms of coffee)
Moldy or spoiled foods
Salt
Yeast dough
Garlic
Broccoli (in large amounts)
Hops (used in home brewing)
Rhubarb leaves
Pear pips, the kernels of plums, peaches and apricots, apple core pips (contain cyanogenic glycosides resulting in cyanide posioning)
I guess the bottom line is that you should feed your dog Dog Food. 
Now that I've educated myself, perhaps this thread needs to be 'stickied'.
Every dog I've had when I was younger have had dog food. Now that I have my new puppy, I would like to expand her diet. I do add carrots and she loves that, but what is the "norm" for adding meat into their diet? Is raw meat poisinous to them? What about seafood, like fish, calamari, clams, etc?
