Bitterants are used in antifreeze to prevent pet and child poisonings. It is required by law in some places (France, Oregon, etc.)
Gas dusters often use a bitterant to discourage inhalant abuse, although this can cause problems for legitimate users. The bitterant not only leaves an intolerably bitter flavor in the air, but also leaves a bitter residue on objects, like screens and keyboards, that may transfer to hands and cause problems (such as when eating).
Examples of bitterants
Denatonium is used in a variety of applications as an aversive agent.