Christine in "Beauty and the Opera, or the Phantom Beast," by Suzy McKee Charnas. The Phantom's whole idea of morality consists of obeying Christine's decree that he must not murder people who annoy him.
Niko is this to Cal Leandros as his human side slowly starts dwindling away. He is also just as protective of Niko as Niko is of him. So it's a bad idea to hurt either of them, mm'kay?
In The Demon's Lexicon by Sarah Rees-Brennan, the protagonist's brother Alan acts as a Morality Chain for him.
Dexter's foster father, Harry, who trained Dexter to select his targets according to certain standards of morality and justice. Dexter calls it the "Code of Harry".
Subverted in later seasons, when the show becomes less clear on whether Dexter was actually by nature a murderous sociopath with no ability to control his instincts, or whether he was just traumatized and schizoidal, and whether Harry just misread the situation and manipulated him for the sake of pursuing personal vendetta.
Totally averted with all his girlfriends except Rita, and played with in regards to his stepdaughter Astor, because he will kill to protect her without going through his usual vetting process.
Turned on its head in Witches Abroad. Granny Weatherwax is good because her sister turned evil, which by the Theory of Narrative Causality meant she had to be the 'good' one to balance things out. Granny never forgave her sister for that, since she maintains adamantly that she'd be an infinitely better villain than the sister. It should be noted that Granny was more pissed off because she never got to choose. She had to be the Good One because her sister was the bad one. That the sister thought she was the Good One was simply the icing on the cake.
Witches as a "community" keep an eye on each other for this very reason. Any group of witches larger than Three has a tendency to fall apart from bickering, but there's still a silent understanding that some level of contact helps them avoid "going to the bad".
Zigzagged with Snape in Harry Potter: though his love for Lily was not enough to keep him from joining the Death Eaters, Voldemort threatening her life switched him to good, and her death (and his responsibility for it) was the main reason he dedicated his life to protecting Harry, even though the two loathed each other.
Downplayed with Aragog and Hagrid. Aragog's loyalty and devotion to Hagrid were strong enough to suppress Aragog's natural instinct as a dangerous magical creature to eat humans. This only went so far; Aragog didn't try to deny his family the opportunity to eat Harry and Ron when they visited him in the Forbidden Forest, though he would prevent them from trying the same with Hagrid. Of course, whether or not a sapient monster (whose instincts drive it to eat people) actually eating people could be considered "evil" is a tricky matter.
In I, Claudius, Tiberius has three people like that: his first wife, Vipsania, his brother Drusus and his friend Cocceius Nerva. Claudius notes that initially their influence checked the worse elements of his nature, but as he was forced to divorce Vipsania and Drusus was sent on a military campaign to a different part of the empire, their influence on Tiberius was removed and he gradually went altogether to the bad (especially after the two died). As for Nerva, he was too absent-minded and innocent to keep Tiberius in check.
Relatively benign example in Luke Skywalker and the Shadows of Mindor. Nick Rostu was mind-controlled into attacking Luke, who took a chance and saved Nick. Now Nick can return the favor by gunning down thirty-odd mind controlled people and saving Luke... but... he hesitates
because he had an overpowering intuition: if Luke Skywalker thought he might save thirty innocent lives by sacrificing his own, he wouldn't hesitate. Ten innocent lives.
"Or, hell, one not-so-innocent life," Nick muttered. "Like mine." He flipped the carbine's power setting to stun. "I hate Jedi."
In the Matthew Swift series, the titular character is this for the blue electricangels who are infantile, amoral, quite capable of burning London to the ground for shits and giggles, and with whom he shares a mind; although others suspect he may be more of a Morality Pet. On the occasions when Matthew is forced to take a back seat, havoc ensues. As one character puts it:
Vera: God, if there wasn't a fucking sorcerer still in that skin, they'd have ripped the city apart just for kicks.
Later in the series, when Matthew becomes Midnight Mayor, it's speculated that he was chosen for the office (as London's mystic guardian) in order to force the angels to be invested in the city's well being, essentially making the entirety of London into a Morality Chain.
In the novelization of Metropolis, Hel functioned as one of these for both Joh Fredersen and Rotwang. After she dies, Joh withdraws completely into work and becomes a cold-hearted executive, and Rotwang withdraws into his laboratory to spend all his time plotting revenge and building a robotclone of Hel. At the end, Joh Fredersen is redeemed by his son Freder, who becomes his new morality chain.
"One Lonely Night" begins with Private DetectiveMike Hammer angsting over being chewed out by a judge merely for blowing away a scumbag who likely would have been executed anyway. He spends much of the book bothered by his Honor's assertion that he's no better than the man he killed, and so is quite pleased when he's able to retrieve the MacGuffin without killing many more people. Then his Sexy Secretary Velda gets kidnapped and Mike realises he's been kidding himself. Of course he's an evil murdering bastard. That's what he was put on Earth for! He then proceeds to track down the Dirty Communists who kidnapped his secretary and Kill Them All.
In the Prince Roger books by David Weber and John Ringo, Sergeant Nimashet Despreaux is told by a number of her comrades that she must survive because she is the Morality Chain for Prince Roger MacClintock, who will be the next emperor of mankind. She is also told that is why she has to marry him.
The Reynard Cycle: Persephone is this to Reynard. Which is rather ironic, given that the majority of the reprehensible things he does throughout the series are motivated by him wanting to possess her.
After his death and Brain Uploading, Nahrmahn plays this role for Merlin in the Safehold series. He offers reassurance when Merlin fears what his power (to essentially kill anyone he has to with impunity) may turn him into, and makes sure Merlin doesn't shut himself out from his friends and loved ones.
The second half of the original Slayers novels (the ninth-through-fifteenth novels, which were never translated into English) introduced a new rival to Lina, Luke. He is an ex-assassin, and the only reason he never turned back is because of his partner Milina (and it' shown that he is clearly in love with her). With her he is more or less a Jerk Ass, but when Milina is killed later on, Luke becomes a crazed Omnicidal Maniac, allowing his piece of Shabranigdo, the Big Bad, to awaken out of his soul.
While the cold and ruthless Tywin Lannister of A Song of Ice and Fire was never really a nice guy (he wiped out two families of rebellious vassals as a young man), it's mentioned that he used to be somewhat more humane, and in particular, showed his most visible happiness and emotion around his beloved wife. When she died in childbirth, all of that humanity was gone for good.
In The Warrior's Apprentice, the sadistic Sociopathic Soldier Sergeant Bothari has two morality chains: his daughter Elena and the young Miles Vorkosigan. Lacking any moral sense, he uses duty to dictate how he treats them.
War and Democide Never Again. The protagonist, John, thinks he is this to Joy, but when things get hairy he is completely ineffectual in reining in her sociopathic tendencies... until she crosses the Moral Event Horizon completely, leaving him with no choice but to kill her to prevent her from causing any further harm.
Invoked in the Warhammer 40,000 novel Deus Sanguinius. When Rafen offers to challenge Arkio's claim to be Sanguinius reincarnated in combat, Stele is glad despite the unexpected turn Rafen's survival has caused, because he believes that Rafen is the last link Arkio has to his humanity and that Rafen's death will cause Arkio to be permanently lost to Chaos.
Catherine Earnshaw is this to Heathcliff in Wuthering Heights. He was never a nice person (though this may be a case of Then Let Me Be Evil, considering that everyone ostracizes him and expects him to be evil from the start because he's an Ambiguously Brown rescued street urchin), but once she marries another man and then dies in childbirth, he develops into a near-demonic Villain Protagonist and takes elaborate revenge on everyone who has ever kept him from Catherine or otherwise screwed him over (and theirchildren).
The Hunger Games: Peeta Mellark. Played up subtlety in Catching Fire. By the end of Mockingjay, Katniss explicitly states this is why she ultimately chooses Peeta over Gale.
In The Kingkiller Chronicle, there are signs that Kvothe is a morality chain to his Poisonous Friend Bast. As a Prince of The Fair Folk, Bast has at best an academic understanding of human morality, but cares deeply for Kvothe's well-being and follows his counsel — aside from the things he gets up to behind Kvothe's back "for his own good."
Urban Dragon: Arkay has zero compunctions about murdering anybody who so much as disrespects her, but she tones it down so as not to upset Rosario.
Deconstructed in the backstory of Brimstone Angels. The tiefling cleric Alyona was the only person her twin sister Bisera cared for, and she kept Bisera relatively grounded in her quest for magical power. When Alyona was murdered, Bisera became obsessed with resurrecting her, and in so doing she became far worse than she might have been otherwise, to the point of bargaining with Asmodeus himself, performing wicked rites to help him rise to full godhood in exchange for Alyona's life, and ultimately changing her name to that by which she would be known and feared for generations: Bryseis Kakistos.
In The Spirit Thief, Josef is the reason Nico hasn't gone off the deep end yet. She's a demonseed, always at risk that a demon will take her over, but she has Undying Loyalty for Josef and his words - and sometimes, even his presence, or someone mentioning that he needs her help - are often enough to pull her back from the brink.