Chiming in late on the Toxic Carnival, but I can't help but mention one of the most historically significant chemical threats to life on this planet.
My chosen chemical is arguably responsible for one of the greatest environmental catastrophes in history, whose effect was global in its reach and certainly changed the biological face of the planet. Said chemical is still produced in vast amounts to this very day, and no one seems inclined to do anything about it.
The culprit, of course, is dioxygen (O2).
Billions of years ago (a bit over 2 billion of them, in fact), the Earth's atmosphere became significantly more oxygenated due to the increasing extent of oxygenic photosynthesis. I can't even begin to fathom how many otherwise innocent anaerobic bacterial species must have been driven to extinction. Of course, it was perhaps just a matter of time once oxygenic photosynthesis evolved from its anoxygenic roots.
The worst part is that oxygen still wreaks havoc among organisms to this very day. Exposure to higher-than-normal partial pressures of oxygen can be toxic, and - in fact - many animals have elaborate mechanisms of oxygen transport that serve to protect the organism from unfettered oxidative damage, including specific "oxygen chaperones" (in essentially all vertebrates, this role is filled by hemoglobin). Of course, anaerobic organisms are still susceptible to the threat of dioxygen in their environment to this very day. The reasons for this can range from insufficient amounts of enzymes capable of metabolizing reactive oxygen species (catalase, peroxidase, and others) to oxygen poisoning their (frequently novel) catalysts - err, metalloenzymes - that are specific for anaerobic metabolism.
At a more molecular level, dioxygen is critical for the function of cytochromes P450, which has been termed "nature's blowtorch." That doesn't sound very soothing, now does it? One of the oxygen atoms is doubly reduced and scoots off as water, leaving behind a vicious biological oxidant which will insert the remaining oxygen atom even into fairly unreactive C-H bonds. Reactive oxygen species such as superoxide and peroxide are produced as a result of oxidative phosphorylation, due to incomplete reduction of dioxygen by the cytochrome c oxidase complex.
When you take a deep oxygen-rich breath one of these days, think of the poor anaerobes who can't. You should feel a twinge of guilt.
6 hours ago