Wulfgar, the barbarian from the popular Drizzt novels (based on the Dungeons & DragonsForgotten Realms setting), seems to fit into this type in almost every fight he's in after being resurrected after a decade of torment by Errtu. Unable to cope with the hopelessness and torture he endured, and fearing that his escape is all some dream, he fights recklessly. Several times, Drizzt and others have had to divert their tactics to save him from himself.
Skandians from Ranger's Apprentice series can (unintentionally) enter sort of a berserker state, when they gain super-strength and supernatural resistance to pain and wound. And become Ax-Crazy. Most of them doesn't survive the experience, though.
Fitz, protagonist of Robin Hobb's Farseer and Tawny Man trilogies, tends to go into a "battle haze" whenever he fights, disregarding his own safety to savagely lay about himself. This makes him very effective during battle, but then he's left standing around delirious for a few minutes until the berserker-mode wears off.
In Robert Low's The Whale Road one of the Viking ship's crew is a skinny man with a bad leg named Pintel. Throughout the story the main character, Orm wonders why no-one ever mocks Pintel over anything and why the man is even able to mock their Captain while anyone else is threatened with death. Later on a newer Christian member of the crew pulls down Pintel's offering to Odin and in the ensuing argument mocks Pintel's leg. Pintel challenges the man to a fight. At the beginning of the duel Pintel throws away his shield and begins to froth at the mouth. Pintel then leaps onto the challenger and hacks at him until there is very little left other than a lot of blood and some blocks of flesh.
Also in the Forgotten Realms are the dwarven Battleragers who love combat and jump into it with a glee that scares their allies as much as the enemy. Considering that their fighting style incorporates armour that can only be described as a mobile cheese grater which is used to shred opponents by rubbing against them with furiously, this is probably justified.
Grigoriy Pechorin, the Byronic Hero of Mikhail Lermontov's A Hero of Our Time, has extreme ennui for a fatal flaw and so lives by this principle as well. He leads charges on enemy positions, enters a duel he knows to be rigged and volunteers to tackle an Ax-Crazy drunken Cossack.
Logen Ninefingers from Joe Abercrombie's The First Law series is a pretty tough fella normally, but when he's desperate and driven to extremes, he completely loses control and becomes "The Bloody-Nine," an unstoppable killing machine who holds no distinction between "friend" and "foe," only "dead" and "soon to be dead." Logen hates this Superpowered Evil Side of himself and spends a good portion of his life trying to escape it.
Touchstone goes into rages that give him frightening power and disregard for things such as physical impossibilities (i.e. trying to hoist up a throne affixed to the floor so he can throw it at someone). He regrets these bitterly, and they are said to be the result of his mother's affair with a warrior from the North. It is revealed towards the end of Sabriel that this is why he was frozen as a figurehead of a ship for 200 years.
His son, Sam, inherits this trait to a degree. Perhaps due to his more cautious disposition, he never loses himself to the extent that Touchstone does.
The title character of the prequel Clariel also has this as does her mother, Jaciel, and her rages are explored in more depth; she tries to use various techniques learned from a book on berserkers to try and keep a lid on it, but the wild, unrestrained passion of the berserker rage also gives her a natural affinity for Free Magic that will, eventually, turn her into Chlorr of the Mask.
The Badger Lords, and anyone else unfortunate enough to have the bloodwrath, from Brian Jacques' Redwall series. When the bloodwrath takes over, the warrior will throw themselves into battle, seeking to reach and kill their mortal enemy (their eyes glow red, even in darkness, and one point-of-view shows that everything around them kind of disappears into red mist with only their archenemy appearing clearly). They are completely heedless of their own safety, and will kill anyone—friend or foe—who tries to get in their way or otherwise stop them. They usually end up killing scores of foes, their mortal enemy, and themselves.
The Lord of the Rings: In the Battle of Pelennor fields, King Theoden of Rohan gets the narrative description: "Fey he seemed, or the battle-fury of his fathers ran like new fire in his veins" Later, when his nephew Éomer finds Theoden's body together with his unconscious sister and believing her to be dead as well, he turns into a full-on Death Seeker.
"Maedhros did deeds of surpassing valour, and the Orcs fled before his face; for since his torment upon Thangorodrim his spirit burned like a white fire within."
"Last of all Húrin stood alone. Then he cast aside his shield, and wielded an axe two-handed; and it is sung that the axe smoked in the black blood of the troll-guard of Gothmog until it withered, and each time that he slew Húrin cried 'Aurë entuluva! Day shall come again!' Seventy times he uttered that cry; but they took him at last alive..."
Beorn in The Hobbit plays this quite literally, being a "skin-changer" who can take on the form of an actual bear in battle. He wins the final battle by going Straight for the Commander.
In James Swallow's Warhammer 40,000 novels Deus Encarmine'' and ''Deus Sanguinius, the Blood Angels are perpetually tempted by their "flaw", the "red thirst", which transforms them into this when they succumb. Stele unleashes it in opponents to be rid of them. At the climax, Rafen succumbs to this; on the other hand, it does unlock the powers of the Spear of Telesto for him, and the daemon he fights is shocked to see that the many futures in which Rafen failed instantly vanish. Then the spear protects him. When the dying daemon unleashes it in the other Blood Angels, they terrify their enemies, who retreat although they never retreat, and the spear even, astoundingly enough, lets Rafen bring back his battle brothers who had succumbed.
David Weber's War God's Oath series features a species of these. It's somewhat involuntary, and they're not happy about that.
Erik Hakkonsen from The Shadow of the Lion series is capable of this, but since he doesn't know friend from foe once he enters a rage, he refuses to do it when the prince he's guarding is nearby.
In Wraith Squadron, the multiminded alien character "Runt" had a "pilot mind" who was a berserker. As berserkers make very bad pilots, Runt did badly enough to qualify for the Wraiths. With his wingman's help Runt eventually got over this problem.
In The Warlord Chronicles, Derfel muses several times about how any man, whether he be a justice loving generous soul like Arthur or a loving family man like Derfel can transform into a monster in battle, especially when victory seems likely.
''A terrible hate wells up in battle, a hatred that comes from the dark soul to fill a man with fierce and bloody anger. I knew that Saxon shield wall would break. I knew it long before I attacked it. The wall was too thin, had been too hurried in the making, and was too nervous, and so I broke out of our front rank and shouted my hate at the enemy. At that moment all I wanted to do was kill... so I ran ahead, madness filling my soul and exultation giving me a terrible power as I picked my victims. They were two young men, both smaller than me, both nervous, both with skimpy beards, and both were shrinking away even before I hit them. They saw a British warlord in splendor, I saw two dead Saxons.
Also his description of the Irish Blackshields, an army of berserker soldiers and raiders.
The Blackshields did not attack in a line, but came in a howling mass. This was the Irish way of war, a terrifying assault of maddened men who came to the slaughter like lovers.
Galbatorix creates groups of magically modified soldiers who cannot feel pain in the third book of the Inheritance Cycle. They disregard their safety because they can take crippling injuries and continue on, making them a whole army of berserkers.
Roran Stronghammer once killed nearly 200 men in a single battle and when he starts fighting is said to experience a battle rage that lends him strength and allows him to overcome many obstacles. Though he also still relies on smarts and cunning.
Jarek of The Seventh Tower. Milla almost turns into one when Tal sells her shadow in Aenir but this trope is defied when she uses meditative breathing exercises to calm herself down.
In the Starlight and Shadows trilogy by Elaine Cunningham one of protagonists is a berserker. Since the abilities are considered partially magical, when Toril's magic was messed up, he became a little closer to the original — that is, his rage sooner or later started spontaneously in any fight, then he lost all control and usually fought until no standing opponents present (though he still accepted unambiguous surrender in this state). Since both he and his superiors knew soon he's likely to lose ability to tell foes from allies, this was a problem.
Boïndil from Dwarves, to the point that he mistook his wife for an orc and killed her while raging. And he's a good guy.
Conan himself is a natural berserker. As put in Queen of the Black Coast, "The fighting-madness of his race was upon him, and with a red mist of unreasoning fury wavering before his blazing eyes, he cleft skulls, smashed breasts, severed limbs, ripped out entrails, and littered the deck like a shambles with a ghastly harvest of brains and blood."
Many of Howard's other characters are also born berserkers, including King Kull and Solomon Kane, all the more unnerving with the latter because of his otherwise ironclad self-control, like in "Wings in the Night" when the akaana slaughter his new friends: "Kane laid the body gently down, looking for Kuroba. He saw only a huddled cluster of grisly shapes that sucked and tore at something between them. And Kane went mad." Kane gone mad is frightening as Hell.
According to Audie Murphy's autobiography To Hell and Back, there were two occasions when he started firing an oversized gun, cursing, and giving no regard to his own safety. He says that the memories of these events are very fuzzy, like it was a dream.
Among the many, many werewolves of The Dresden Files are lycanthropes, humans who don't actually shapeshift but maintain a pack mentality and have major rage issues. When the full moon comes around, they go feral and have to hunt something down.
A Harry Dresden whose instinctive sense of decency has been outraged is a truly fearsome sight - and the last for anybody or anything that tries to stand against him.
When Harry fully embraced his mantle as Winter Knight he became nearly animalistic in combat. He soon realized that the ability to ignore his wounds and predatory mindset were actually more dangerous to him than his enemy and managed to regain control.
Rhodry Maelwaedd, most notably during his time as a silver dagger. Rhodry has an extended metaphor about his love for "Lady Death".
One of Jill's past lives, the warrior woman Gweniver, was like this as well. Both Gwen and Rhodry are referred to as "chortling" in battle.
Jill herself went berserk exactly once. It scared her so much she never did that again.
The Rifter: John when his Rifter self is unleashed. He becomes a divinely-powered whirlwind of fury, smashing everyone and everything in his path; he charges straight into armies, taking innumerable wounds, whose pain fuels his rage and which heal instantly. He is almost as dangerous to friends as to enemies, especially when he shatters buildings and creates gaping chasms in the earth.
With her grizzly bear morph, Rachel from Animorphs is a particularly apt example of this trope. She's been known to use her own severed arm as a weapon in the heat of battle.
In Gregor and the Prophecy of Bane, Gregor learns he's a "rager" a person with highly developed warrior skills, a natural-born killer. Meaning he has a natural capacity to go into this state when his life is in danger. It's not always voluntary, which is really bad for a character who normally hates to kill.
Ripred is also a "rager".
Hurt her husband or her son and Amelia Peabody becomes something far more elemental than an English lady. Watch out for the parasol.
In the world of The Icelandic Sagas, a warrior in berserk-fury is not only supernaturally strong and ferocious, but may also be invulnerable to fire and regular weapons. The sagas' outlook on berserkers and berserking is very ambiguous, because the older pagan tradition saw these behavior or ability as a good thing, while the Christian Middle Ages viewed it either as mere superstition or as evil pagan magic. As a result, when berserkers (berserkir) appear in the sagas, they often do not actually fit this trope: Often, they are merely bandits and troublemakers who live by robbing and blackmailing people and who never actually prove the supernatural ferocity that they claim to possess; the implication is that they just want to scare people into submission with their bogus "berserk powers". Consequently, the sagas will usually avoid calling (especially heroic) characters that display actual berserking behavior "berserkers", presumably because the term is so strongly connected to the aforementioned villain stereotype. Some examples of saga characters that go on berserk rampages without ever being called "berserkers":
But after a while the berserkergang began to come on Valgard, he trembled and frothed and gnawed the rim of his shield, he rushed forward howling and slaying.
Harry Harrison and Tom Shippey's The Hammer and the Cross trilogy features a realistic Nordic berserkr as a major supporting character. When not fighting, he's brooding and melancholy, prone to fits of heavy drinking. When fighting, he's a Death Seeker. One of the main characters notes that all "true" berserkrs are inherently Death Seekers.
Taur Urgas of Belgariad. He's the Ax-Crazy King of Cthol Murgos, and The Brute of the series' Five-Bad Band. He's a total mental case who sleeps in his armour, is prone to fits of madness during which he does everything from commit murder to chewing the furniture, and completely gives into his rage in battle, actually frothing at the mouth as he leads his troops into battle. His madness gives his men a peculiar sense of invincibility, and when he dies (while demanding his enemy come back and fight) their spirits are completely broken.
Barak is a heroic version, who literally turns into a bear and flips out when Garion is in trouble. Bear in mind that Garion pretty much exists solely to get into trouble for the greater good.
Adus of The Elenium becomes this towards the end. Already an Ax-Crazy mentally-handicapped Psycho for Hire, he loses his mind during the climax, to the point where he cuts his way through his own troops to get at Kalten.
''Who's Afraid of Beowulf?'' features Starkad the Berserker, the sweetest, kindest if rather dim hero you'd ever want to meet - outside of battle that is.
The Koloss from Mistborn are a race of berserkers. They're used as shock troops by The Empire and were in fact originally created for this purpose, but in the 1st book the protagonists don't have to worry about them since they're operating La Résistance in The Empire's capital, and the koloss's blood rage means they can't ever be allowed near major population centers you want to leave standing. In the 2nd and 3rd books after the Lord Ruler, who was essentially restraining them telepathically, dies, the koloss start wandering around in hordes and killing anyone they can get their hands on. Eventually the new Big Bad, Ruin, takes command of them.
Sam Vimes exhibits some of these qualities, especially in the climactic battles of Night Watch and Thud!. In Night Watch, Vimes taps into his long-buried rage (that he calls "The Beast") and lets it out as he grabs two swords and hacks his way through the enemy, described as "he wasn't an enemy, he was a nemesis." In Thud!, he appears to be doing much the same thing, to the point of ignoring a dwarfish flamethrower being used on him. Although in that case, it was also a case of possession as he was under the influence of a quasi-demonic thing of pure vengeance called the Summoning Dark.
Subverted in Thud! with Mr. A. E. Pessimal - a small man with very shiny shoes who attacks a troll ... with his teeth. It doesn't end well for him.
In The Generalist you have both Frank and Dash, through different means. In Frank's case, if he gets pushed too far beyond his limits of control, or if his rage takes over, or if his quirk of nature "Overkill" gains control of his body, then he can fly into an utter Berserker rage. Dash, on the other hand, has a far greater chance - each time he uses the ability Maximum Troll, his chance of slipping into a Monster-gene fueled berserker fury rises.
In Taboos 2 and 3, he actually does lose it, forcing Frank to hit him with the Roadbuster. The first two times broke him out of Maximum Troll, the third was just for fun.
The Iron Tower has Danner Bramblethorn, a Warrow berserker. During the last battle of the trilogy he charges all of the Goblins on the walls, ranting about how he is "King of the Rillrock."
In A Harvest of War Wild Rhona claims to be able to enter a berserkergäng without drugs. She doesn't seem to need it much, though.
In the Starlight and Shadows trilogy, Fyodor is a berserker from the land of Rashemen, which is well known for its warriors being able to enter a magical berserker state. Fyodor, however, is unable to control his berserker rages, and is exiled from his homeland due to the danger he poses to his allies.
The Witchlands has an understated version of berserkerism called the Nihar rage, which affects members of the Nihar family. They're extremely Hot-Blooded, cursed with Hair-Trigger Temper, and more prone to physically attacking someone. When Vivia feels it for the first time, she notes that it makes her fearless, focused and gives a boost to her witchery, and Merik implies that there's some low-level magic to it.
In The Dark Profit Saga, berserkers are members of the Brotherhood of Flame, the most elite of Dwarven warriors. Everyone assumes they "get so angry they go crazy in battle," and it certainly looks like it, especially since they have a hard time remembering the details afterwards. According to Gorm, though, purpose, not anger, is key (although anger has its place too).
Gorm Ingerson: Ye find something in the battle to fight for, something ye'd die for. Your brothers back in the clanhome, the honor of your Da's name, the lives of innocents. A reason to fight, if nothing else, like a tiny fire, and ye reach out and grab it. And ye hold it no matter how it burns. And soon ye can't separate yourself from your purpose, any more than ye could take the light from a candle flame. Ye live to win. Ye can't lose; ye can only die.
Incarnations of Immortality: In Wielding A Red Sword, Prince Mym fights like this when he tastes blood. Thanks to his incredible training and discipline, he retains his fighting skills and can direct his rage at his true opponents.