Pets should avoid:
- all grapes, raisins, currants, etc. are potentially hazardous and shouldn't be given to your dog.
- of course, chocolate is toxic to dogs in large amounts...
- onions, chives (& some people say garlic should be avoided too)
- caffeine
- tomatoes
- avocados
- seeds and pits from fruit (they contain cyanide, thus the grapes)
- sugar and corn syrup
- nutmeg
- all nuts (their levels of phosphorus are said to cause bladder stones) exception: peanut butter - that's good for dogs (just use salt and sugar free peanut butter)
- cold cuts in large amounts are bad for dogs due to nitrates and salt content
- alcohol
- Xylitol - a sweetener used in gum and baked goods.

Of course if you drop a small piece of onion here and there, or garlic or any of the above you're not going to harm your pet, but I find it to be useful information nonetheless.
There's a lot of info out there on this subject > click here for a google search on the subject if you'd like to learn more.