Xylitol is a natural substance found in fibrous vegetables and fruit, as well as in corn cobs and various hardwood trees like birch. It is a natural, intermediate product which regularly occurs in the glucose metabolism of man and other animals as well as in the metabolism of several plants and micro-organisms. Xylitol is produced naturally in our bodies; in fact, we make up to 15 grams daily during normal metabolism.

While Xylitol might be good for humans, it is toxic in dogs.  Xylitol is a sugar alcohol used in candy and chewing gum. It is also found in some pharmaceuticals and oral health products such as chewable vitamins and throat lozenges. It can also be used in home baking.

http://www.snopes.com/critters/crusader/xylitol.asp

http://www.aspca.org/site/DocServer/vetm0207f_095-100_.pdf?docID=10462

All grapes are potentially dangerous to dogs, both grapes and their counterparts, raisins, and regardless of whether they came from the store or off the neighbor's vine. Don't feed your dog grapes or raisins, and don't leave these foods out where your dog(s) could help themselves.

http://www.snopes.com/critters/crusader/raisins.asp

http://www.dogbreedinfo.com/grapeandraisin.htm

http://www.peteducation.com/article.cfm?cls=2&cat=1939&articleid=2409

Poisoning of dogs by chocolate is not uncommon.

http://www.talktothevet.com/ARTICLES/DOGS/chocolatetoxic.HTM