Just because it may be good for humans doesn't mean it's good for our four legged friends!
Candy (particularly chocolate—which is toxic to dogs, cats, and ferrets—and any candy containing the toxic sweetener Xylitol)
Coffee (grounds, beans, and chocolate-covered espresso beans)
Gum (can cause blockages and sugar free gums may contain the toxic sweetener Xylitol)
Hops (used in home beer brewing)
Onions and onion powder
Potato leaves and stems (green parts)
Tea (because it contains caffeine)
Tomato leaves and stems (green parts)
Xylitol (artificial sweetener that is toxic to pets)
This information was compiled from various sources.