Fist of Rage

So your plucky hero has decided that enough is enough and has retired from the scene, looking to return to a life of mundanity and safety. Unfortunately, the villains just can't leave well alone and the next thing you know, the Hero's girlfriend has been kidnapped/house burned down/dog has been shot, and the Hero watches the wrong-doers ride off laughing into the distance.

Cut to the hero's hand, hanging limp by his side. In comes the Theme Music Power-Up as the hand slowly curls into a fist. The fist quickly turns white with intense squeezing - maybe it even trembles with determined strength. He's Back Cue the Roaring Rampage of Revenge!

The dramatic clenching of a fist is often used as a visual signal that the character's resolve to perform the upcoming series of heroics is hardening, or as a signal that they have regained something - physical or mental - that they needed but were previously lacking. Also done during climactic fights - the villain seems to have the upper-hand and stands gloating over the Hero, who then manifests a Fist of Fury and returns to the battle with previously unseen strength.

Occasionally paired with a Power Fist, and invariably done even though the character's powers and abilities (if any) might not be related to their hands, or even physical prowess.

Truth in Television - It is a common trait for someone who is frustrated or anxious to clench and unclench their fists as they try to decide upon a course of action. While not necessarily a 'furious' version, a clenched fist in real life will often reflect the closing of an internal conflict, even if violence is not directly about to ensue.

Compare Knuckle-Cracking, Angry Fist Shake, Fist of Enthusiasm.

Examples of Fist of Rage include:

Anime and Manga

Dragonball Z is a subversion of sorts, as usually such characters clench their fists BECAUSE of the exhertion required to channel their incredible amounts of power. Played straight by Goku when fighting Frieza, and by Gohan when fighting Perfect Cell however.
Neon Genesis Evangelion: In an interesting usage, throughout the series there are repeated shots of Shinji's hands as his fingers curl as if trying and failing to make a fist. In the final episode of the TV series, as he finally declares "My life is worth living here!", he makes a real fist. The walls of the dark "playroom" around him shatter, to reveal him surrounded by the rest of the cast, all congratulating him under a blue sky.
A Certain Magical Index: Touma always clenches his right hand into a fist before he says he will shatter someone's illusions.
L of Death Note does this after his first defeat, as he resolves to show his face to the task force. (Light also does it in the Rewrite movies - not to reflect any real emotion, just as part of one of his many Bastardly Speeches.)
Guts from Berserk does this surprisingly not during his Screw Destiny speech at the end of the Golden Age Arc (which led into his Roaring Rampage of Revenge), but during his My Greatest Second Chance declaration at the beginning of the Retribution Arc.

Comic Books

In All Fall Down, when Pronto hears about Siphon, he has one of these that's so intense he makes his palm bleed.

Film

Tony Stark does this just prior to unleashing the Mark I armour upon his captors in Iron Man.
Spider Man 2 - Just after Doctor Octavius kidnaps Mary-Jane, the recently retired Peter Parker watches the villain carry her off and seemingly 'reactivates' his ailing powers with this gesture.
A wounded Superman Returns, and then does this seconds before plunging into the Atlantic and lifting the Kryptonite-infested Island into the upper atmosphere.
Jean-Claude Van Damme likes doing this, especially visible when fighting the Sumo Competitor in Bloodsport.
Back to The Future, of course! George does this to Biff in order to protect his future wife.
Ash getting his chainsaw back in the pit at the beginning of Army of Darkness. Chainsaw of fury?
- Not sure about that one - probably closer to Power Fist in both use and appearance - although the scene where Ash gets a steampunk hand and uses it to crush a metal goblet later in the same film would fit.

"Groovy." -Ash

Rocky V - "Touch me and I'll sue!" *Clench and.... WHACK!!!* "Sue me for what!?"
A particularly extravagant example comes from the Incredible Hulk movie (certainly the 2008/Ed Norton film, and quite probably the Eric Bana and Lou Ferrigno versions too). Not only does Bruce Banner clench his fists tightly as he prepares for battle, but those fists start getting bigger and greener....
Bruce Lee's characters did this a lot, especially in the actual Fist of Fury film.
An interesting inversion of this trope happens in Fight Club. When Norton's character is being threatened by his boss, he looks down at his hand and is surprised to see it clenched into a shaking fist. Moments later, he unleashes a brutal beatdown ... on himself.

Literature

Played straight in The Stand by Stephen King, after the explosion that injures Franny and kills and wounds a bunch of others. Stu tells her that Harold set the bomb, and then gets a revenge-hungry look on his face that outright frightens her and does the Fist of Rage with both fists.

Music

Don McLean's "American Pie" contains a line "Oh, and as I watched him [Devil] on the stage, my hands were clenched in fists of rage..."

Web Original

Doctor Steel is often posed in his propaganda art making one (and it's part-and-parcel of his trademark salute), and does it as he's suiting up for "Experiment Time" in The Dr. Steel Show, Episode 2.
The Subgenii faction of Fenspace have sculpted an entire asteroid into one called "The Stark Fist of Removal" after a concept espoused by the real-world Church of the Subgenius.

Western Animation

Zuko from Avatar: The Last Airbender does this every once in a while, especially in the first two seasons, either as he's about to attack Aang or as he's trying to keep his bottled-up frustration inside.

Other

One of the virtues espoused by The Church of the Subgenius is called The Stark Fist of Removal.