Chloral Hydrate is a depressant, specifically used for inducing sleep. Its chemical formula, near as I can tell, is C2H3Cl3O2. It is noteworthy for being the first substance developed specifically for the purpose of a soporific, or sleep aid; it was introduced to public use in 1832. It was successful at its task, and was quickly adopted by the medical profession for use both as a sleep aid and as a general-purpose sedative.
Its most famous use, however, is probably that of one of the active ingredients of the "Mickey Finn," where it is known as "knock-out drops" (or the Bartender's best friend). Mixed with alcohol, chloral hydrate produces a swift deep sleep - and as such, was popular with anyone wishing to (mostly) harmlessly knock out an unsuspecting victim who was drinking. Bartenders used it on rowdy or violent patrons; law enforcement, criminals and date rapists used it on drinkers the world over.
It's still produced today. Although its use has declined, it is still prescribed for insomnia as well as a post-surgical sedative and pain relief for adults and children when mixed with painkillers such as aceteminophen (paracetamol for those in the British Commonwealth). There are more effective concoctions available these days, but the old school still soldiers on. It's available in capsule, syrup and suppository(!) form, in the U.S. under the brand AquaChloral.
National Institute of Health
Thanks to filoraene for a correction on the molecular formula!
An unnamed noder says: "When I worked at the old folks' home, chloral hydrate was a favorite to use on combative grannies. At least until Mellaril came along."