"You're sorta stuck where you are, But in your dreams, you can buy expensive cars, Or live on Mars and have it your way. And you hate your boss at your job, But in your dreams, you can blow his head off. In your dreams, show no mercy."
Sometimes people can be really mean to you. The Jerk Jock and the Alpha Bitch will mock you in school, the Sadist Teacher and Obstructive Bureaucrat won't leave you alone, the vile new boss can turn your beloved workplace into hell on earth. Even your parents can hurt you. And sometimes your whole city will hate youfor some reasons. And you know what the worst part of it is? You can't take revenge. You can't tell your boss what you really think about him, unless you want to lose your job. Standing against the bully will just get you a serious beatdown for your trouble. And how the hell are you going to fight with the whole town? Sometimes the only thing you can do is give in to your imagination. Because there, you can be anyone — The Chosen One, a Super Hero or just Badass with a cool longcoat. And everyone who ever pissed you off will have to pay. You can give villains the faces of your abusers, or just imagine them being beaten by you or your avatar.
The Sliding Scale of Idealism Versus Cynicism shows two possibilities of this trope played seriously. In more cynical settings, a person stuck in this situation will never get to stand up against his abusers, always living under their heel and either will become a completely broken person as time passes or just cracked up. In more idealistic settings that person will at some point stand against those who turned his life into a living hell, either verbally or with fists.
Sometimes, this can be played for laughs: Sam pissed off Bob and Bob imagines himself beating the tar out of Sam, but this was just a separate incident — Sam and Bob are friends, or at least don't have a reason to hate each other.
Most definitely Truth in Television.
See also I Just Want to Be Badass, arguably the same basic desire; Dream Sue, where the characters imagine themselves to be ridiculously perfect; and Wish Fulfillment.
Isao Kako in Bokurano has a disturbing dream in which he takes revenge on everyone who was mocking him, including beating up his older sister alongside two bullies and trying to rape the Alpha Bitch.
The black suit of armor from After School Nightmare embodies this trope; in the dream world, it acts for the real-world character as a way to do this in dreams.
One episode of Ghost in the Shell: Stand Alone Complex involves Section 9 looking into the daydreams of a refugee from the last world war who fantasized about killing his boss and rescuing the girl as an escape from his mundane life as a helicopter pilot and society at large. The episode itself is an homage to Taxi Driver, but in the end it is concluded that the man poses no actual threat to society, and that his daydreams are just that.
Lt. Barclay does this in his first appearance on Star Trek: The Next Generation but unlike most other examples he has the holodeck to make his power fantasies reality.
TV adaptations of Shredderman Rules, which are for the most part faithful to the book.
Played for laughs in The Young Ones in the episode "Summer Vacation," where Neil imagines himself turning into the Incredible Hulk and attacking his housemates. While none of that actually happens, he does actually take his shirt off, much to Vyvan's confusion.
Calvin has three different imaginary alter egos — Spaceman Spiff (which provides the header picture), Stupendous Man, and Tracer Bullet — of this trope as major recurring characters, plus other fantasies of being a dinosaur.
There's also the strip where Calvin fantasizes about being an all-powerful, sadistic god who enjoys tormenting the denizens of his little world. The final panel reveals that he's playing with Tinker Toys.
FoxTrot: Peter gets these whenever he is left in charge of his younger siblings, including fantasies of being an all-powerful god.
Beetle Bailey once takes the advice to think of something nice during a long march to make the time pass faster. He has so much fun imagining abusing an unresisting Sarge that he doesn't even notice when the march is over.
Arguably most tabletop role playing games. Most of these games let you play some larger than life character doing heroic things. Though there are occasional subversions such as Paranoia. The Munchkin is an example of someone who lets this desire overcome their sense of fairplay.
Countless video games fulfil a power fantasy. This is especially true for modern video games produced in the early 21st century. Aside from most puzzle games, some survival horror games, some abstract action games, and a few other exceptions, modern video games in general are based off this trope.
This was only partially true for older video games in the late 20th century. While older games also provided a limited power fantasy, they were generally a lot more difficult, requiring more work and dedication from the player, and with death usually meaning Game Over. Modern video games have generally become a lot easier, with very little consequences to death, in order to provide a greater power fantasy to the player, without much difficulty getting in the way.
Mewt in Final Fantasy Tactics Advance breathes the trope. In the real world, he is painfully shy at school and he is constantly picked on and bullied by the other children. Mewt's mother passed away some time ago and his father is an emotional wreck who barely gives Mewt any emotional support. When he and his friends get thrown into the fantasy world of Ivalice, Mewt becomes a prince, his mother is alive again and is a queen, his father is the a Judgemaster, he has a personal assistant that will tend to all of his needs, no one bullies him anymore, and everyone does what he tells them to do. Only when Mewt listens to his friend, Marche, about standing up for himself and being true to his self does he give up the power fantasy and agrees to go home.
Metal Gear Solid (1998) deconstructs the trope. The game constantly reminds the player that being a Super Soldier that can kill dozens or even hundreds of generic men and that war isn't as fun or glamorous that other forms of media may make it seem to be and that you will always lose people that you grow attached to during war. Most players missed the point and saw the game as nothing more than a power trip with Solid Snake being so badass for taking on a tank, a helicopter, Metal Gear itself, and a slew of characters with special abilities.
Metal Gear Solid 2: Sons of Liberty (2001) deconstructed the trope much further, and was made to specifically attack the audience that thought being a badass during a war or hostage situation was so fun. It replaced the badass Solid Snake with an amateur Raiden, in many ways meant to represent the player. The game often mocks the player, with various comments directed at Raiden also directed at the player. At times the game even punishes the player, rather than rewarding them, after beating a boss or clearing an objective. This only got worse as the player progresses through the game, with the Colonel at one point mockingly telling the player/Raiden to turn off the game console. Read more about how it deconstructs the power fantasy trope here.
The trope is also deconstructed by Spec Ops: The Line, a game which looks like it will fulfil this need on the part of the player. For the first hour, it even imitates the military shooter ubiquitous in the late 2000's and early 2010's. Then it becomes clearer and clearer that by wanting to be the "hero" of the story, the Player Character is worsening the situation, which was already disastrous and desperate. Some of the loading tips are brutally sarcastic and suspiciously ambiguous about whether they refer to the protagonist or the player. One developer stated that the ultimate answer to the moral quandaries the player encounters is to stop playing, thus letting go of the expected Power Fantasy.
John Konrad: The truth, Walker, is that you're here because you wanted to feel like something you're not. A hero.
Done hilariously in Justice League Unlimited. When Dr. Milo, a high-ranking Cadmus employee, is told by his boss (Amanda Waller) he's fired, he at once pulls out a huge laser cannon and kills everyone else at the conference table. Of course, it's a fantasy and in real life he just meekly takes his pink slip before releasing Doomsday as payback.
In the Fairly OddParents, Timmy Turner has created so many imaginative alternate personae that the rest of the town starts asking him which one he is each day when they notice he's not looking or acting like his usual self.
Angela Anaconda has these Once an Episode... One time she even paid homage to Digimon Adventure to the point where, in an altered version, she actually mentions Digimon terms, like Digivolve and attaching -mon at the end of people's names.