When we took our dog to puppy class, the trainer suggested we put peanut butter inside a kong as a treat when we go out. He's two years old now but I still put a smidgen of pb in his kong.
My veterinary surgery presently have a warning up in reception. Some (not all) peanut butter contains something called Xyletol. Xyletol is a chemical used in sugar free gum as a sweetener and it can be deadly to your dog.
If your dog eats xylitol, this sugar substitute enters its blood stream much faster and ,unlike your body, your dog’s body does not recognize that xylitol is not true sugar. The dog’s pancreas then releases large amounts of insulin to deal with it. This can cause a sudden, severe drop in your dog’s blood sugar level (hypoglycemia), similar to what diabetics experience if they take too much insulin.
http://www.2ndchance.info/Xylitol.htm
I came home and looked at the label on my peanut butter and was relieved to see it had no Xyletol in it. Its more likely to be in sugar free peanut butter. I then went online to look up what other foods contain Xyletol and the list was alarmingly long, so before you give your dog a bit of your left over food, make sure it doesn't have Xyletol in it.