Flunitrazepam (Rohypnol, roofies)

• fast-acting and powerful sedative/hypnotic drug used primarily to treat insomnia and settle people down prior to giving them general anaesthesia
• is a benzodiazepine, meaning that it acts on GABA-A receptors, which are prevalent throughout the brain, to enhance the flow of chloride ions into neurons, resulting in a generalized depression of neural activity that results in sedation (feeling dopey) progressing to hypnosis (sleep)
• is about 7-10 times more potent than diazepam (Valium), the archetypal benzodiazepine
• all benzos also work as anxiolytic and anticonvulsant agents, as well as skeletal muscle relaxants
• widely used recreationally to enhance the effects of alcohol and other drugs, although some people get high off of it alone (mechanism is unknown) or use it as a 'downer' to deal with the side effects of stimulants ('uppers')
• has been used as a date rape drug and to facilitate robberies (it causes anterograde amnesia in addition to sedation, so the victim often remembers nothing) and may precipitate violent behaviour in certain people
• long-term use can result in psychological and physical dependence (i.e. you can get addicted to it)
  

Druid H, Holmgren P, Ahlner J. Flunitrazepam: an evaluation of use, abuse and toxicity. Forensic Sci Int. 2001 Nov 1;122(2-3):136-41.
http://en.wikipedia.org/wiki/Flunitrazepam