A thought maker was a device designed to control the neural activity or "brainwash" the humanoid brain, making the subject highly susceptible to suggestion. It was illegal to use or even possess one of these devices within the Ferengi Alliance.
In 2364, Ferengi DaiMon Bok used these devices on Jean-Luc Picard in an attempt at revenge for killing his son years before. Bok made Picard believe that he was again captain of the USS Stargazer, and that the USS Enterprise-D was an attacking enemy vessel. He had purchased two of these devices from a world not yet discovered by Humans, and it cost him the profits of a lifetime. (TNG: "The Battle")
Deanna Troi later described it to Commander Dexter Remmick as a "mind altering machine" that controlled Picard without his knowledge; Remmick took that to mean that Picard had a "mental lapse". (TNG: "Coming of Age")
In the Star Trek: The Next Generation - Intelligence Gathering issue "Matters of the Mind", a thought maker was used to control Deanna Troi, Doctor Crusher, and Ro Laren by Romulans in order to try to direct the Enterprise-D to a new destination.