In Hookie Dookie Panic, Adda pulls a few of these, the biggest being when she loses a Dance Dance Revolution tournament due to a fight going on, and she proceeds to go into a psychotic rampage and kill those responsible for the fight.
In Sam & Fuzzy, Mr. Blank has his moment of Unstoppable Rage against Mr. Black, at least in the beginning. The funny (or creepy) thing is, the audience could already sense the doom months ago.
Averted in Sluggy Freelance. Torg appears to go into one of these after Horribus kills Alt-Zoe, but, in a rare moment of rationality, he realizes saving the world is more important than a grudge match, and runs away instead.
Girl Genius has this for its Sparks. When they get in the Mad Place, they unleash all their most devastating gadgetry and can throw around clanks single-handedly. However, it is said that this is also the reason for most lower level sparks' deaths. They get in a rage without the ability or sense to keep themselves from dying.
Tarvek flies into one of these while fighting Zola, beating her to the point where she tries to surrender — only Tarvek isn't listening to silly notions like that.
Agatha goes full tiltafter Lars is killed. Thank goodness for Sparky pies.
Dr. Jean Poule in The Inexplicable Adventures of Bob! can go from adorable Girl Next Door innocence to blind fury without too much trouble. This rarely manifests as physical violence, as the mere force of her yelling is usually enough to produce results (i.e., Bob: "Wow! Thanks, Jean! You're swell at yelling at people!" Jean: "Well, the secret is to enjoy it...") but it has gotten her into fights upon occasion.
In Slightly Damned, Buwaro was born with permanent Unstoppable Rage. The only reason he's so happy-go-lucky is because his star pendant keeps the rage in check.
Aradia Megido is first introduced as a depressinglyEmotionless Girlbecause she's a ghost, but later on when she gains a new robot body she becomes capable of showing one in particular. Rage.
Karkat Vantas walks around in a state of perpetual ire and will verbally tear into anyone with little provocation, so it's kind of a subversion that he doesn't do this.
Gamzee Makara's title is the Bard of Rage. From all accounts, it manifests as him being nigh-unstoppable when he gets going - he is said to have dealt the most damage to the Black King with just one strike, and that was when he was high. Whenhe'ssober, this trope is more or less his default state. To wit: what he did to Nepeta...Eventually however, Gamzee's rage is stopped by Karkat via a Cooldown Hug.
Rose enters one after learning that Bec Noir had killed her mother. Seeking revenge, she well and truly loses it when he kills John at the very beginning of the battle, prompting her to destroy a large percentage of the battlefield in an attempt to kill Bec Noir. This being Homestuck, Rose's attempt is in vain and she eventually gets killed (and to add to insult, she did no damage to Bec Noir.)
Bec Noir goes into this after Jade dies again, and slaughters most of the Trolls' Dream Selves in their session.
John Egbert, despite his usual calm personality, get into that state two times:
After he was already enraged by Davesprite, being more of a jerk, than usual, he meets Jack Noir and proceeds to beat him so furiously, that he resorts to blocking flurry of attacks, unable to fight back. Jack has to bind him with tentacles, just to get a chance to hit back. And even this didn't work. Just in one hit John manages to harm him nearly as much as Rose through the entire fight and cause tremendous damage to his reputation in PM's eyes..
His second meeting with Caliborn he unleashes all hatred, built trough their previous encounters. At first Caliborn putting up a good fight, but soon enough ends up on the floor, receiving one punch to the face after another. Only thing, that stops John, is his abrupt teleportation out of the fight, by which point Caliborn lies on the ground, looking borderline traumatized.
After Slick breaks God-Tier clock, Doc Scratch finally gets fed up with his behavior, and proceeds to beat him into a pulp (with a teach-yourself programming book,nonetheless) and punch him through the wall, sending him fall a several stories to the ground.