Denatonium
Denatonium, usually available as denatonium benzoate (under trade names such as Bitrex or Aversion) and as denatonium saccharide, is the most bitter compound known to date. 
It was discovered in 1958 during research on local anesthetics by Macfarlan Smith of Edinburgh, Scotland, and registered under the trademark Bitrex. Dilutions of as little as 10 ppm are unbearably bitter to most humans. Denatonium salts are usually colorless and odorless solids but are often traded as solutions. They are used as aversive agents to prevent accidental ingestion. Denatonium is used in denatured alcohol, antifreeze, nail biting preventions, animal repellents, liquid soaps, and shampoos. It is not known to pose any long-term health risks although exposure may be irritating and unpleasant.
An amusing anecdote relates how one of the researchers at Atomergic Chemetals Corp. in Plainview, New York, went home without realising that he had a tiny trace of denatonium saccharide on the outside of his lip. When he kissed his wife, she almost vomited.

# Structure and physical properties
Denatonium is a quaternary ammonium cation. It is a compound of a salt with an inert anion like benzoate or saccharide. The structure of denatonium is related to the local anesthetic lidocaine, differing only by the addition of a benzyl group to the amino nitrogen.

# Applications
The bitterness of the compound guides most applications of denatonium. Denatonium benzoate is used to denature ethanol so that it is not taxed as an alcoholic beverage. One designation in particular, SD-40B, indicates that ethanol has been denatured using denatonium benzoate. In fact, the common name for this chemical, denatonium alludes to this application.
Denatonium also discourages consumption of harmful alcohols like methyl alcohol and ethylene glycol. Denatonium is therefore often used in rubbing alcohol as an inactive ingredient. It is also added to all kinds of harmful liquids including solvents, paints, varnishes, toiletries, and other household products.
Since 1995, when the State of Oregon required that denatonium benzoate be added to antifreeze and windshield wiper fluid, the compound has been increasingly found in these substances throughout the world. The addition is credited with saving children and animals who might otherwise drink sweet antifreeze or wiper fluid and get ethylene glycol or methanol poisoning respectively.
Other uses include nail polish for preventing nail biting, and as animal repellent (especially for big game like deer).
Non-human animals are known to have different sensitivities to the effects of denatonium. It has been used to safeguard rat poisons from human consumption, so presumably rats are not deterred by it, although there is evidence that a small percentage of rodents do avoid such baits.  Some cats have been known to be tempted by them   it may not be as effective a deterrent for cats as it is for humans, or perhaps some cats are not deterred due to a genetic factor similar to that affecting human perceptions of the taste of phenylthiocarbamide.
It is soon to be used on all Bindeez toys in response to the incident where two children experienced a coma after ingesting the small beads.  It was later discovered that it contained a chemical which metabolizes to GHB, a drug with alcohol-like effects in small doses.  Though that problem is unlikely to reoccur, the bitter substance will be used to prevent future ingestion and choking.