Cocoa Mulch, which is sold by Target, Home Depot, Foreman's Garden Supply and other garden supply stores contains a lethal ingredient called Theobromine (see below).  It is lethal to dogs and cats.  It smells like chocolate and it really attracts dogs.  They will ingest this stuff and die.  Several deaths already occurred recently.

The doting owner of two young lab mixes purchased Cocoa Mulch from Target to use in their garden.  The dogs loved the way it smelled and it was advertised to keep cats away from their garden. Their dog (Calypso) decided the mulch smelled good enough to eat and devoured a large helping. She vomited a few times which was typical when she eats something new but wasn't acting lethargic in any way. The next day, Mom woke up and took Calypso out for her morning walk. Half way through the walk, she had a seizure and died instantly.

Although the mulch had NO warnings printed on the label, upon further investigation on the company's web site …this product is HIGHLY toxic to dogs and cats. 

Cocoa Mulch is  manufactured by Hershey's, and they claim that "It is true that studies have shown that 50% of the dogs that eat Cocoa Mulch can suffer physical harm to a variety of degrees (depending on each individual dog).  However, 98% of all dogs won't eat it."
*Snopes site gives the following information:http://www.snopes.com/critters/crusader/cocoamulch.asp *  

Theobromine is in all chocolate, especially dark or baker's chocolate which is toxic to dogs. Cocoa bean shells contain potentially toxic quantities of theobromine, a xanthine compound similar in effects to caffeine and theophylline. A dog that ingested a lethal quantity of garden mulch made from cacao bean shells developed severe convulsions and died 17 hours later. Analysis of the stomach contents and the ingested cacao bean shells revealed the presence of lethal amounts of theobromine.