Grimlock has had several of these in Transformers Meta and will probably have several more.
He raged against Barricade and Soundwave in the third episode when they were holding Bumblebee captive.
He beat the heck out of Barricade in the second episode when Barricade nearly took out Bumblebee.
Furthermore, he charged furiously at Slipstream when Bumblebee was, yet again, being held captive.
In the same episode, he went absolutely crazy when Bumblebee had been kidnapped and went so far as to beat up a corpse because there was no one else around that he could kill.
In the second episode, he tried to snap Wheeljack in half when the latter left Evac and Bumblebee by themselves in a potentially hostile envrionment.
Inevitably, Grimlock's Unstoppable Rages are commonly caused by, but are not limited to, something happening to Bumblebee.
In a My Little Pony / The Dresden Files crossover fic My Little Denarians, the ponies get exposed to much more non-cartoon violence than Harry would have liked - and they learn fast. At one point Derpy is shot by a sniper, causing Dash to charge across the street at the attacker. She may be a cartoon pony but she is a supersonic cartoon pony - after running across the street and up the stairs, Harry finds the room splattered with blood and bits of flesh and the furious Dash Pummeling the Corpse
Kyon Big Damn Hero. If you do anything to harm the SOS Brigade, Kyon - or if you hurt him, Haruhi - will lose it and you will suffer.
In the Firefly fic Forward, Jayne flies into one of these when River takes a bullet for him during the assault by Niska's goons. It is described as Jayne "seeing red." Jayne periodically has these moments throughout the story, when his anger overrides good sense. Later on, River goes into one of these rages when a Hand of Blue shoots Simon, and in the resulting scene reduces his face to hamburger by repeatedly stabbing it in a hysterical, savage frenzy.
Logan Wright from Dalton has some rather serious anger issues, causing him to violently lash out at virtually everyone on a semi-regular basis.
Blaze from The Legend of Spyro: A New Dawn enters this when the curse preventing him from helping his daughter fight the Warden is broken. He shoots the Warden a Death Glare, plows straight through everything between him and his target (including the cage he was in and a mob of Gargoyles), and sends the Warden flying clear across the room twice in a row. He then leads the former slaves and his daughter in a No-Holds-Barred Beatdown on the Warden, ending it by blasting the Warden through a wall. Do not mess with Blaze's daughter!
Sleepwalker will explode into this trope when confronted with his Arch-Enemy Cobweb, who trapped him on Earth, or his Evil Counterpart Psyko, who murdered the parents of his Heroic Host Rick Sheridan. Sleepwalker's anger typically pushes him beyond his normal limits when dealing with Psyko, although even then the result is a bloodbath from which Sleepwalker typically emerges more dead than alive. An unusual twist on this trope is that Cobweb has also played on Sleepwalker's anger to manipulate him into losing focus and making bad decisions that would get him killed...at least until Rick serves as a Spanner in the Works and sets Sleepy straight.
When Violet discovers Ludlow's true motives in Episode 4, she basically snaps. And spends the remainder of the fic turning this trope Up to Eleven, which is more than enough to tip the balance of power in the protagonists' favor.
In The Wizard in the Shadows, these happen to several characters, most frequently Harry. The usual sign of Harry having been in Unstoppable Rage mode is a large crater. One caused by a previous incident of Rape Is a Special Kind of Evil was one hundred yards across. The size of a soccer pitch, in other words.
Harry when he finds out that Saruman has been torturing Sirius for the best part of seven years. He throws Wormtongue from the top of Isengard, catches, him, gives him to Éomer, drags Saruman down into the dungeons by his beard(after letting Emrys break his nose and forcing it back into joint), vaporizing the doors of Isengard, and when he finds Sirius he loses his temper completely and repeatedly uses the Cruciatus curse.
Also, Emrys doing his level best to drown Wormtongue after finding out that Wormtongue raped his sister. Theodred has to call him off, much like the Batman example above.
Harry when Ginny is poisoned by the Witch-King, unleashing the full power of a Patronus infused thunder storm. Part of the weakest bolt vaporises a Mūmak.
Ginny when Harry dies unleashes three enormous tornadoes of fire on the army of Mordor, destroying the physical forms of three Nazgūl in the time it takes to snap your fingers.
Pages Of Harmony involves Fluttershy going into this at one point. She's had Kindness stripped from her so completely that she hallucinates that her pets are the ponies she believed wronged her, and tears them mercilessly to pieces. She then turns this attack on Twilight, whose Well-Intentioned Extremist actions were the cause of her present suffering, and they pummel one another into a bloody mess, with Fluttershy tearing chunks off of Twilight's skin before Twilight eventually manages to win by a margin.
In the RWBY fic Massages, Team CDRL beats up Jaune as usual, but when they reveal that they plan to rape every girl in his Unwanted Harem, he loses it and gives them a No-Holds-Barred Beatdown. The others are forced to knock him out, not out of concern for Team CRDL, because he was about to burn himself out from releasing so much energy at once.
'No fighting policy' be damned. Morals and the law didn't mean anything anymore. Jaune would kill these motherfuckers.