Sodium Pentothal is known as a 'truth serum', and also for its use in execution by injection. The FBI is currently considering using it on the four suspects held on charges related to the attack on the WTC, who have so far refused to speak1.
Sodium Pentothal is the trade name for Thiopental Sodium aka Sodium Thiopentone (C11H17N2NaO2S ), which is a heavy tranquiliser. It is also used in executions by injection, as detailed in SockMonkey's writeup in Lethal Injection Execution.
Thiopental Sodium is a barbiturate that makes the neural membrane more permeable to chloride ions. In a small dose, its effect is to remove the inhibitions of normal behavioural restraints. It also relieves tension and anxiety, and has been shown to make people talkative. Higher doses result in a stupor and inhibit independent thoughts and actions - and the person becomes more suggestible and less willful. At higher doses it can knock a person unconscious.
The effect of lower doses does not force the recipient to tell the truth. It does make telling a lie more difficult - lying takes more mental effort than telling the truth. The drug has been used in drug-assisted hypnosis. It is highly unlikely that any evidence obtained with Sodium Pentothal would be admissible in any court of law.
Sodium Pentothal has uses besides interrogation and execution2.
Thiopental Sodium was first discovered by Ernest H. Volwiler and Donalee L. Tabern in 1936, when they were looking for an injectible substance that could produce unconsciousness. They discovered that it could be used for minor procedures requiring anesthesia or for more prolonged procedures inconjunction with other substances (such as Ether)
A 20ml injection of a 2.5% solution will cause unconsciousness within 15 to thirty seconds, and last from five to ten minutes. Today it has three main uses:
As an induction to anaesthesia. Pentothal is used to put the patient under quickly and easily, and then the more effective (normally respiratory) anaesthetic is then applied to make sure s/he stays under for the duration of the operation.
By itself for short periods of time. This is best for minor operations.
Pentothal also has its uses as an anticonvulsant, which can be used (for example) by people caring for epilepsy sufferers. In this context, it is quick and effective, though limited in duration of action.
Note on spelling: There's pentothal, and pentathol. The manufacturer (Abbott) uses the former: http://animalhealth.abbott.com/product/pentothal.htm3.
1: The report with the FBI considering truth serum (among other things) is at http://www.washingtonpost.com/wp-dyn/articles/A27748-2001Oct20.html
2: pointed out to me by Miles_Dirac
3: pointed out to me by Professor Pi
Thanks to schist for pointing out spelling errors.