The mind-sifter (also known as a mind-scanner, and sometimes dubbed mind-ripper when used more forcefully) was an interrogation tool and weapon used by the Klingon Empire in the 23rd century. The device could be used either to scan surface thoughts, or to record and extract all thoughts from a subject's mind. At its most dangerous settings it would leave a subject in a vegetative state. (TOS episode: "Errand of Mercy")
In 2376, a connection between the mind-sifter and the mind controlling powers of the ancient Malkus Artifacts was discovered by Klingon Defense Force Lieutenant Toq, leading Lieutenant Commander Data to theorize that the mind-sifter may have been based on Zalkatian technology uncovered on former worlds of the Zalkat Union within the then Klingon Empire. (TNG - The Brave and the Bold, Book Two novella: The Final Artifact)
The ancient Vulcans that participated in the Exile from their homeworld were able to create a prototype mind-sifter based on the technology that was used to create the Coronet. (TOS - Vulcan's Soul novel: Exiles)
A variation of its application was used on the Vulcan brain, in 2252. Shran interrogated Soval to determine the Vulcans' true motives for Andor. To which his brink of mental control was reached, and broke Soval's otherwise logical barrier. (ENT episode: "Cease Fire")
In 2267 the Klingon occupation force on Organia subjected Spock to mind-sifter interrogation to establish if he was telling the truth about being a trader (as opposed to a Federation spy). Thanks to his Vulcan mental control techniques Spock was able to maintain his story even at force four, a level usually able to "break down any pretence", and was subsequently permitted to go about his business, the Klingons sure he was telling the truth.
When Spock and Captain Kirk's (who had been passing as an Organian) true identities were revealed Kor, son of Rynar, the leader of the Klingon force, threatened to use the mind-sifter on Kirk - though was not able to carry out his threat before Kirk and Spock escaped. (TOS episode: "Errand of Mercy")
Later that year, Spock was again subjected to the mind-ripper after being captured by Klingons. His conscious mind retreated deep into his memory, and after he was rescued, Dr. McCoy and Captain Kirk used the mind-sifter they also retrieved to enter Spock's mind and draw him back out. (TOS short story: "Chaotic Response")
Some time later the Klingons used the mind-sifter on Romulus to prove to the new Praetor, Gaius, that another Romulan had been part of a conspiracy to kill the previous Praetor and Gaius' mother and implicate the Klingons in the act to destabilise the Klingon-Romulan alliance. (TOS comic: "The Hollow Crown, Issue 2")