In the mirror universe, an agonizer was a device developed and used by the Terran Empire's Imperial Starfleet to punish their members for relatively minor offenses. It was invented by Dr. Sarah April in the 23rd century, and its first human test subject was Captain Nathan Thorpe. Its initial tests against Klingons proved to be a failure. (TOS short story: "Ill Winds")
Agonizers were worn on a person's belt, and could be demanded by any superior officer at any time if they felt a subordinate had committed an infraction. When the agonizer was applied to the body it would cause intense pain which could bring a person to their knees. (TOS episode: "Mirror, Mirror")
In 2370, Arctus Baran used neural servos, which were implanted in each member of his crew, as a means of maintaining obedience. (TNG episodes: "Gambit, Part I", "Gambit, Part II")
By the 24th century, agonizers were refined to the point that they no longer needed physical contact with their victims to be effective.
In an alternate version of the mirror universe, agonizers were still used by the Terran Empire in the 24th century, and had been refined to the point that they were incorporated into the Starfleet insignia worn on a person's uniform. As Deanna Troi was head of ISS Enterprise-D security, a special agonizer was designed specifically to function with Geordi La Forge's VISOR. (TNG novel: Dark Mirror)
In the primary universe, an agonizer-like weapon was standard issue to Klingon personnel in 2268. One was used by Kang's landing party to deter an outraged Pavel Chekov on Beta XII-A. (TOS episode: "Day of the Dove")