Mr. Dark: Your torments call us like dogs in the night. And we do feed, and feed well. To stuff ourselves on other people's torments. And butter our plain bread with delicious pain... Funerals, bad marriages, lost loves, lonely beds, that is our diet. We suck that misery and find it sweet.
The Feasters in Lady Of Gems series feeds off fear.
The titular dream eaters of The Dream Eaters and Other Stories are the mythical baku, creatures which feed on nightmares and negative emotions. Their disappearance causes the dragons of The Dream Eaters to suffer bad dreams.
The White Court vampires of The Dresden Files are all psychic vampires that draw power from other people's emotions. The vampires of House Raith draw power from lust, and are incubi and succubi down to the last; the vampires of House Malvora draw power from fear; and the vampires of House Skavis draw from despair, and usually try to drive people to suicide so they can feast.
Thomas Raith presents an interesting angle on the feeding issue; his lover, Justine, suffers from derangement and schizophrenia. Thomas' feedings act as a form of psychic therapy and keep her condition under control sans the negative side effects seen with medications. Without him regularly feeding on her, she rapidly slips back into madness.
When Thomas is trying to be a Friendly Neighborhood Vampire, he finds work as a hairdresser, because emotional intimacy can be used as a substitute for lust, and requires taking less life force.
The plot of Proven Guilty brings us more Emotion Eaters in the form of phobophages, monsters who mimic the form of a subject's fears then feast on the tasty, tasty terror. The ones featured in the book draw from the mark on the national consciousness left by horror movie monsters and wreak bloody havoc on a horror convention.
The Skinwalker is also made more powerful by fear, and by inflicting pain.
Speaking of vampires, Robin McKinley's Sunshine dealt with this too. Tears stood in as a weaker substitute for blood, and anguished giving was more powerful but ultimately more destructive than 'natural' sacrifices.
This is the reasoning behind the plot of the Wicked Lovely book Ink Exchange. The faeries of the Dark Court feed off the negative emotions of other faeries, and are weakened by times of peace and happiness. In order to avoid fading away entirely, its king devises a method of feeding off of the negative emotions of humans.
Dementors drain happiness and warmth from their surroundings, and can make someone relive their darkest memories. For this reason, they're kept as guards in the wizard prison of Azkaban, so that the prisoners can't work up the will to escape. You can fight off their effects with chocolate (chosen because Dementors' effects are like depression, which is weakened by chocolate) or creating a Patronus; a projection of your happiest memory which at its strongest takes the form of an animal spirit.
You can also get around them two other ways, more indirectly. If you're an Animagus, your emotions are simpler in animal form, so you won't suffer as much. They also only eat happy memories, so plans centered around unpleasant facts won't be disrupted. This was how Sirius Black escaped; he took the form of a dog, and wanted revenge for being sentenced for a crime he didn't commit.
The Wizarding equivalent of the death penalty is the "Dementor's Kiss," which sucks out your soul. Lupin describes it in Prisoner of Azkaban as becoming an Empty Shell - you just "exist" with no memory or sense of identity.
Vampires in the Anita Blake series have secondary feeding methods, usually relating to drawing nourishment from a person's emotions.
The Spectres from the His Dark Materials trilogy drain their target's attention, turning them into mindless, apathetic bodies.
In the lastThe Dark Tower novel, Roland and friends find that Dandelo a.k.a. Joe Collins is one of these. Of course, the irony is they are warned several times to beware of the individual. The individual hides using Glamour.
The Wild Cards series has a few psychic "Aces" who draw power from certain rituals packed with emotion. Fortunato, for instance, draws power from the passion of tantric sex, while The Astronomer draws his power from the ritual torture and sacrifice of young women.
The Stephen King novella The Library Policeman focuses on an inhuman thing posing as a librarian that feasts on the fear of little children.
Also from Stephen King, the novel It focuses on an inhuman thing posing as a clown that feasts on the fear (and flesh) of children.
The Hunter from the Coldfire Trilogy feeds on fear, despair and other negative emotions.
While his is essentially parasitic, the Iezu in that universe each have a specific emotion they feed off of in a more symbiotic way. One has set up shop as the god of Pleasure, and feeds while his followers enjoy themselves. And one has set up shop as the god of Sadism, and nearly becomes the most powerful entity in the world. It's also a significant plot point that these creatures are so polarized that the opposite of their emotional attunement weakens or destroys them. Pleasure, of course, is defeated by apathy, while sadism is finally undone by altruism.
Sergey Lukyanenko's Night Watch (Series) puts an interesting spin on this trope. As expected, the evil Dark Others draw their power from negative emotions like fear, anger, hatred, etc, but essentially make people more "good" by removing these feelings from them. Meanwhile, the good Light Others who draw their powers from positive emotions like hope, love, faith etc. leave the world a darker place as they basically suck joy out of the universe to power their magic. As such, neither side tends to be enthusiastic about seriously draining the energy of people around them. Unless, you know, the fate of the entire world rests in the balance (which seems to happen at least once every few weeks).
The situation still is to the advantage of the Dark Ones though, since for the most part they don't actually care about making the world more evil. They're mostly interested in their own power, which means that they can draw as much power as they think they can get away with.
As well as the Others, the first level of the Twilight is home to a blue moss which gains nourishment from emotion on general. Light Others kill this with fire, Dark with ice.
Occasionally, a Light one snaps and goes on a rampage taking Power from the happy people around him. Why? In order to destabilize the status quo and make the world a better place. Naturally, the Night Watch usually tries to stop them, as this gives the Dark ones a blank check to do something bad. In New Watch, one does so by going to a crowded soccer stadium and feeding off the energy of the fans of the winning team. Apparently, he amasses so much Power that not even a Mage Beyond Categorization can easily stop him. Anton uses the young mage's inexperience (the mage casts a shield that protects from any spell, instead of one to protect against spells and physical damage, which is expressly stated to be a rookie mistake most mages outgrow and fast) to knock him out with a baton.
Banshees from The Hollows novels feed on negative emotions of others even unto death. In a disturbing recent development one banshee and her child learned to feed off love as well making them a double threat.
In Nina Kiriki Hoffman's Spirits that Walk in Shadow, there are the viri. Most viri feed off of many different people in a subtle way that causes very little effect, but one viri got addicted to the main character's depression and was making her perpetually depressed in order to feed on that feeling.
Sookie Stackhouse meets a maenad, a wild woman who feeds off of pride and drunken violence (as well as eating meat).
Cyril M. Kornbluth's short story "The Mindworm" has a mutant human who feeds off the feelings of others, killing them in the process. He deliberately tries to induce strong emotions in his victims so that he can get a full meal. Too bad for him that when he starts preying on a community of recent Eastern European immigrants, they find that the old ways of dispatching vampires work just fine.
The vampire nobles in the Sonja Blue series feed on emotions as well as blood, and thus were closely involved with historical events like Stalinism and Nazism.
In The Secrets of the Immortal Nicholas Flamel, vampires feed on emotions. Even blood-drinking vamps (who are considered the weakest class of vampire because of this. Stronger vampires don't need to drink blood to get emotions). This is used by a heroine, with permission, to make a scared and overwhelmed comrade feel better.
S.L. Wright's Confessions of A Demon has immortal shapeshifting demons who can sense and feed off of the emotions of humans as well as other demons. Some of the more benevolent demons will feed on the sorrow and pain from others in order to alleviate their anguish. Each individual demon is associated with a particular emotion that they like to feed on most of all, although they can absorb any emotion. The emotion a demon is associated with stems from what the demon's progenitor was feeling at the time that demon was spawned, and gives each demon a particular aura that makes them inspire that emotion in others. Demons usually name themselves after the emotion they are associated with, such as Bliss, Shock, Pique, etc.
Rehvenge and Xhex of JR Ward's Black Dagger Brotherhood are half-vampire, half-sympath, and a full sympath, respectively. There's even a derogatory term for their sub-species "sin-eater," though they aren't restricted to just feeding off someone's sins (they can devour any range of emotions).
In The 13 ½ Lives of Captain Bluebear, hobgoblins live off emotions such as fear, despair and sorrow. They keep Bluebear alive to enjoy his crying fits.
Tobduk grows stronger by feeding on others' anger as well as his own.
Vamprah's Mask of Hunger lets him grow stronger by feeding off the positive emotions of others.
Stormwings, from Tamora Pierce's Tortall Universe, feed off of fear although they don't suck it out of people. They're not evil — it's simply their nature, as they were created by a human who wanted to deter others from war.
Lord Loss from The Demonata feeds off of human pain and suffering.
Song at Dawn: Estela describes Alienor's Ladies-in-Waiting as feeding off the misery of others (usually each other) and "getting fat on it".
Molassar, the Big Bad of The Keep is this in the original book. He pretends to be "merely" a vampire instead of an Humanoid Abomination Emotion Eater that is older than recorded history. He feeds off all negative emotions, including fear, pain, misery, etc. This allows him to gain strength from both the fear of the soldiers he murders one by one, and to work towards destroying the faith of a Jewish intellectual when he pretends to be vulnerable to the cross. Interestingly enough, Molassar doesn't need to cause the emotions himself to gain strength from them; if he were not trapped inside the Keep, he could feed on all the psychic misery, pain and suffering going on all over the world... and since the story takes place in the early days of World War II, there is plenty of that to go around.
In Pact, bogeymen are former humans cut off from their connections with the world who fell into an Eldritch Location known as the Abyss, and lose their humanity in the process of clawing their way out. Now tied to the Abyss, they require connections with the world to stay in it — and the easiest way to do that is through fear. In addition, these connections feed them power.
In Children of the Black Sun, Sympaths are mages who can draw on human experiences and sensations to fuel magic. Typically, they draw on positive things — the more pleasure around them, the more magic they generate. However, it's possible for Sympaths who are exposed to large amounts of pain to "switch" to drawing on that instead — and Sierra, the protagonist, is such a person. Naturally, the fact that her magic feeds on pain does not make her already magic-averse fellow citizens like her, but she's not actually a bad person.
In Shaman Blues, ghosts gain energy from strong emotions, although it apparently doesn't hurt the person the ghost leeches from.
Kim Newman's Bad Dreams features The Kind, who feed on humanity's emotions and imaginings. Some of them are emotional vampires who specialize in negative emotions; others act as muses to great creative minds. One way and another, they did quite well out of Hollywood.
In Dorothy Must Die (based on The Wonderful Wizard of Oz), the (not-so) Cowardly Lion feeds on others' fear. This has the effect of causing the person whose fear he devoured to age.
In Fate/strange Fake, Jack the Ripper is a Berserker Servant with a Noble Phantasm called "From Hell: The Evil Mist Will Perish With London's Daybreak". This causes Jack to transform into a demon, based on the rumors of Jack really being a demon. The demon form's strength is increased by the latent fears and unease of everyone in a 5 km radius. So if it were used in a desert, it would only be about as strong as a large beast. If used in a city, it would truly be a force to be reckoned with.
The Stormlight Archive: The God of Evil Odium spends a lot of time trying to convince people to give him their guilt and pain, and his Unmade are often described as consuming various emotions. That being said, it's unclear if this is literal or metaphorical. Several people break free of Odium's control despite having implicitly given up their agency, and the final fight between Kaladin and Amaram is all about how Amaram still hurts over all the terrible things he's done.
Huitzilin in the Doctor Who New Adventures novel The Left-Handed Hummingbird feeds on the psychic energy created by terror and panic. But the more powerful he gets, the more he needs to sustain him.
The Grief Leech in the New Series Adventures novel Plague City feeds on negative emotions, although it points out it does nothing to cause them; that's just humans being humans.
The Snata in the New Adventures novel Sky Pirates! feeds on fear and revulsion, and — while mindless — instinctively takes the form of a Santabomination to do so.
Zannies in Market of Monsters eat pain. Severe pain. They don't cause it by being around you, though — if they need to eat, they have to cause it the old-fashioned way.
The Girl Who Drank the Moon features the Sorrow Eater which draws magic from the suffering of others. It establishes social circumstances to inflict sorrow on large populations to farm its food.
In the Siren Novels trilogy, sirens feed off of men's attraction to them. They gain strength by seducing men, and even more strength by luring them into the ocean and drowning them.
Balendin from Chronicle of the Unhewn Throne is an emotion leach, a spellcaster who draws on the emotions that others feel toward him to fuel his magic. To facilitate this, he acts as reprehensible as possible to inspire fear, hate, and disgust: in combat he hurls obscene threats and taunts at his enemies, while for more covert magic he kidnaps innocent victims and tortures them to death like a serial killer. He meets his match in Kaden, who can enter a state called the vaniate where he feels nothing at all, leaving Balendin with nothing to draw upon.
Sweet & Bitter Magic: Tamsin was made incapable of love by a curse, so she takes other people's emotions in return for spells she casts on their behalf.
In Knights of the Borrowed Dark, the Clockwork Three feed specifically on misery, at one point taking over an entire orphanage and trapping its occupants in nightmare-ridden sleep in order to feed on them. They only wake once the Three are killed.
The third book reveals that the Opening Boy has been hiding in Grey's shadow since the above incident, feeding on his depression and PTSD and influencing his emotions and actions.
" has a mutant human who feeds off the feelings of others, killing them in the process. He deliberately tries to induce strong emotions in his victims so that he can get a full meal. Too bad for him that when he starts preying on a community of recent Eastern European immigrants, they find that the old ways of dispatching vampires work just fine.