Thoughtcrime

"Whether he wrote DOWN WITH BIG BROTHER, or whether he refrained from writing it, made no difference. Whether he went on with the diary, or whether he did not go on with it, made no difference. The Thought Police would get him just the same. He had committed -- would still have committed, even if he had never set pen to paper -- the essential crime that contained all others in itself. Thoughtcrime, they called it. Thoughtcrime was not a thing that could be concealed forever. You might dodge successfully for a while, even for years, but sooner or later they were bound to get you."

- George Orwell, 1984, Part 1, Chapter 1

One of the most insidious, brilliant and disturbing concepts is the idea of your ability to think freely being illegal. Whenever a government controls the speech and actions of its citizens, it'll label (what it considers) disapproved thought as Thoughtcrime, and take whatever steps needed to quell Thoughtcrime, by every means capable. One could even call such governments paranoid, if they're willing to go this far, just to keep the citizens from revolting.

If a reason is ever given at all, apart from the obvious, Thoughtcrime can be explained as "intrusive thoughts," and their 'repression' leads to "a happier society." On the other hand, maybe you're in a time of war, and Thoughtcrime represents "impulses to treason." Maybe you're in a state of emergency following a massive terrorist attack, and Thoughtcrime is characterized as "sympathizers of terrorism." As another scenario, you could have been locked into a state of cold war with a rival nation, whose ideologies conflict with yours; Thoughtcrime can be used to justify a Witch Hunt for spies. Maybe there's a civil war erupting between various factions within your nation, and Thoughtcrime is identified as rebellion (as decreed by the ruling faction) or complicity (as declared by the opposition). In some cases, the civil war is more nuanced than just La Résistance against The Empire; whatever they're fighting for, Thoughtcrime can be described as sympathizers or collaborators to whatever ideology the warring factions embody (pro-slavery, anti-slavery; capitalism, communism; isolationism, interventionism). Whatever the excuse, governments (including those controlled by a Mega Corp) use it against its own citizens to suppress dissent, forcefully and effectively.

Note that this is extreme hypocrisy on the part of the guys who came up with Thoughtcrime; they use their free will to censor others' free will. Telling that to them, however, will lead to...

Also note that it's nearly impossible to remove anti-Thoughtcrime policies once enacted, for obvious reasons.

Related to The Evils of Free Will. In more nuanced stories, some of these guys sincerely believe they're using Brainwashing for the Greater Good. For others, it's just business as usual. As a means of propaganda, if the methods combating Thoughtcrime are known to the public, the government (or their corporate benefactors) might attempt to paint it in a lighter vein by calling them Enhanced Interrogation Techniques.

Film

 * THX 1138, which is Nineteen Eighty-Four-esque.
 * Apparently something like this existed to some degree in the world of Starship Troopers, at least in the military. At one point, a soldier says he'd like to get back at the drill sergeant who broke his arm. He's immediately corrected by his fellow soldiers with, "Whoa, improper attitude!"

Literature

 * Trope Maker comes from Nineteen Eighty-Four, by George Orwell. To hammer it home, the main character of the novel, Winston Smith once wrote in his diary, "Thoughtcrime does not entail death: thoughtcrime IS death."
 * In the short story "Harrison Bergeron", thinking is literally outlawed.

Live Action

 * A mundane variant comes from the Doctor Who episode "The Happiness Patrol", where enforced cheerfulness was the law on one planet.
 * Star Trek: Voyager had an episode where they came across a people who were extremely telepathic, so sensitive that any extreme emotions would incite them to act out on those feelings; having violent thoughts was a crime in and of itself. Torres was put under trial for having a brief violent thought when someone bumped into her, and Tuvok's investigation into the planet's culture found a sort of "violent thoughts" Black Market. Of course it examined the nature that when something was so taboo it meant their own people were unable to handle it when confronted with the situation.

Stand Up Comedy
"I live in San Francisco where the parking is impossible. I saw a sign on a guy's garage that said "Don't even think about parking here". So you know what I did? I sat right there and I thought about it. I yelled up at his window "Hey buddy, I'm thinking about it. Go ahead, call the cops. I'll just tell them I was thinking about something else.""
 * Played for laughs in Paula Poundstone's standup routine.

Tabletop RPG

 * Paranoia. Under The Computer's rule every citizen is required to be happy. Anyone who isn't happy is a traitor and can be punished, such as by being required to take drugs that make you happy.

Trope Works

 * Soundscape is particularly vicious at this.

Webcomics

 * Sluggy Freelance's 4U City enforced mandatory happiness with involuntary drugging. And mandatory efficiency with mandatory drugging. And so on. The alternative was to be thrown down a judgement chute. This also created some security holes ("Does it make you happier or unhappier?").
 * StoneToss illustrates the inherent risks of working in thought police: "You Laugh, You Lose".

Western Animation

 * In a merge of Orwellian Editor, Avatar: The Last Airbender has the higher-ups of Ba Sing Se brainwashing everyone who dares to mention that there's a century-long war going on in the whole world outside the walls. The resident Lovable Rogue had this inflicted upon him, which.

Real Life

 * Under the reign of Henry VIII, it became treason to even think ill of the king.
 * At least one street sign seen in New York City in the 1980s read "Don't even think about parking here!"
 * It's still there, a block away from the Burrito Bar.
 * The idea behind "re-education camps" in Communist countries.
 * In the USA, respectful burning is the recommended method for disposing of old flags. However, many people want to ban flag-burning, when the intent is to protest the government. Thus, the actual crime isn't the burning, it's what you're thinking while doing it.