Chocolate

The active molecule is theobromine (C7H8N4O2 - balanced on top of some samples), which is a bit odd because its got no bromine in at all.
Its an alkaloid molecule, and in big enough quantities is poisonous. Most cocoa powder has a range of around 2%-10% of theobromine in it.

As it is an alkaloid, its though this may be behind the chocolate addiction some people get, and may also be where is alleged aphrodisiac qualities emanate from.

Dogs and other animals can't process theobromine as quickly as humans, thus even small doses of chocolate can be poisonous to them, as it builds up in their system to toxic levels. Cats can also be subject to the same, but having no sweet taste receptors are less likely to eat it.

Its also suppose to be very good at suppressing coughs, can cause similar effects as caffeine on wakefulness,restlessness and anxiety, and it probably causes cancer in rats (but then doesn't everything).