The dream seers of X/1999: Kakyou, Hinoto, Kanoe (only in the TV series, in the rest she's more of a Dream Walker), and Kotori
Also, it's revealed that the deceased Hokuto Sumeragi possessed a similar hability, though at a much lower degree; it's hinted that she can only pull this when she's asleep (the others can do it when awake too), and she can enter others's dreams but her influence on them is quite limited. This allows her and Kakyou to meet and fall in love, despite never seeing each other in the real world. It ends in tears.
Actually, Hokuto's twin brother Subaru already showed this skill in Tokyo Babylon. While his speciality was travelling to the subconscious of a cathatonic person like he did to "wake up" Kamui after Fuuma's Face-Heel Turn and Kotori's horrifying murder, in the original TB series he pulled a dream weaver stunt to aid Midori, a girl who had been traumatised due to being raped and later harrassed, thus she fell into a deep sleep to protect herself. And she was his Forgotten Childhood Friend.
The main character from Yumegari, Tatsumi Hojyou. It's actually her family tradition, inherited from her dead parents: her work is to watch over the dreams of other people, and intervene if they're dangerous to the dreamers themselves. Once her mom and dad kick it, Tatsumi sets out to seek for her "destined person" and partner, a man named Kyousuke Kaga who also fits in this trope and lives in Tokyo...
The dolls of Rozen Maiden are capable of entering people's dreams. Suiseiseki is particularly skilled at it, and plays a large part in helping Jun sort out his unconscious thoughts and emotions. Her artificial spirit is even called Sui Dream to reflect that.
Dream/Morpheus from The Sandman, of course. The Norse gods even call him by this particular title.
John Dee, aka Doctor Destiny, an enemy of the Justice League of America. The device that allowed him to manipulate dreams, the Materioptikon, was later shown to have been based on Dream's ruby.
Nightmask from The New Universe and newuniversal. In the former, he's a psychotherapist who uses his power to assist people; in the latter, she's a Japanese-American girl who can now manipulate the "Superflow," the space everyone goes to while dreaming.
Nightmare from Marvel Comics. Morpheus was even partially based on him, visually. The one being he fears above all others, even Doctor Strange, is Gulgol, a monster that never sleeps.
The movie Dreamscape, which involves a villain that can kill people with their own nightmares and the hero, who eventually does the same thing to the villain.
The Golden Child. Sardo Numspa and his minions enter Chandler Jarrell's dream and Numspa controls it to give Jarrell a scary time, including burning his arm so he'll remember the experience.
This is the daily trade of the Extractors in Inception. They create a custom dream for their target which hopefully gets them to spill some closely guarded secret. When the dreamer wakes up, the whole dream will fade, but the extractors remember everything they saw and learned.
Particularly Ariadne, whose job was to build intricate mazes and settings into the dream in order to confuse Fischer's subconscious.
Twice Upon a Time revolves around a plot by the head of the Murkworks (the land of bad dreams) to trick two citizens from Frivoli (the land of pleasant dreams) to helping him bring about endless nightmares for the folks in the Land of Din (our world).
The Dromes of Discworld are like spiders in that they spin dreams instead of webs. If you eat the food in the dream you are trapped until your death, when you will be eaten. They have to wait some time since they have no teeth.
A number of people in The Wheel of Time do this; in fact, there's an entire Dream Land known as Tel'aran'rhiod where dreamers interact with each other.
Charles Render in Roger Zelazny's "He Who Shapes"/The Dream Master is a psychotherapist whose therapy consists of influencing his patients' dreams.
In Anne Bishop's Epherema Duology, it's implied that Incubus and Succubus can manipulate the dreams of others, mainly the Erotic Dreams, but anything with a strong emotional impact will do.
Taen of Janny Wurts' The Cycle of Fire trilogy is actually called a "dreamweaver", but she has a myriad of psychic powers that extend beyond dreams.
In Ryan Graff's The Fires of Affliction, the cult leader known as the Crowning Star has the ability to influence others' thoughts and dreams. At the end of the first book in the trilogy, the Star uses this ability to trap the hero in a dream of perfect happiness, from which he has no will to escape.
Widespread in Labyrinths of Echo. One sequel tells about "Masters of the Perfect Dreams" for a modest price making limited-use pillows with the specific dream wanted by the client. It was legal in Echo even during the strictest limitations on magic and there's a whole guild. The protagonist was visited by a few mages this way. Starting with acquaintance while still living in his (and presumably ours) world and then hiring by his chief. Who as a Professional Killer personally slain about a half of King's enemies during the civil war and later reminisced royal habit of giving such secret decrees, er, friendly requests in dream visits:
Though the old King always paid for the work in waking life. One have to give him credit.
Telepaths in The Whole Man, by John Brunner, can set up shared dreams in a small group. They're called catapathic groupings (a Portmanteau of "cataleptic" and "telepathy"), because nobody involved, including the telepath, is aware of what's going on in the real world, and it can't be broken from the insidenobody wants to leave. Treatment involves another telepath forcing his way into the grouping and mucking it up so badly that the telepath has no choice but to wake everyone up.
Wicked Lovely: Rae can do this; she's referred to as a dreamwalker but it's esencially the same thing. She has been guiding Ani through dreams since childhood, almost destroys faerie by giving Sorcha a dream in which she can see her son, Seth -which causes her to make Rae ensure she never wakes, and it is through her visiting Devlin's dream that him and Ani are able to save Sorcha and thus faerie, and form the 'shadow court' to balance Sorcha's high court.
Devlin and/or Ani also has this ability to a far lesser extent, as they make out in a dreamscape so that she doesn't drain his energy the way she would in reality.
Rae also 'wove' Niall and Irial's dream selves together, which given Irial's eventual death from Bananach's stabbing him means that he might get this ability as well, and so in a way cheating death, but that could just be the fandom's deNIALL kicking in.
The title character in Roald Dahl's The BFG essentially cooks dreams, stores them in jars, then uses a sort of trumpet to blow them into children's ears at night. As opposed to other giants, who simply eat the children.
The mages in Mirror Dreams can create worlds that respond to their mind - dreams or nightmares. However it takes a lot of resources, time and paperwork.
The Silmarillion describes the Vala Irmo (also often called Lórien, after the gardens where he lives) is said to be the master of dreams and visions. The fact that he is the brother of Mandos (the Vala who is almost perfectly able to see the future) is probably the source of prophetic dreams in Arda.
Wild Cards. The Ace named Revenant could send dreams to a sleeping person or even enter their dreams.
Wyrm of The Book of the Dun Cow can use dreams to communicate with the animals aboveground, tempting or harassing them.
In the Majipoor Series, both the Lady of the Isle of Dreams and the King of Dreams have the power to send dreams to any of Majipoor's inhabitants.
In M.C.A. Hogarth's Paradox Universe the telepathic psychotherapists Jahir and Vasiht'h specialize in (and practically invented) dream therapy.
In Rebecca Lickiss's Eccentric Circles, Figwort uses dust on Piper to trap her in an obviously symbolic dream. He appears too, but she can scare him off with the dust that still is in the dream room of her house.
Live Action TV
In the Buffy the Vampire Slayer episode "Nightmares," a little kid in a coma can bring nightmares to life. In "Restless," the First Slayer manages to trap most of the Scoobies in their nightmares.
Heroes featured Sanjog, a mysterious boy who could travel through Mohinder's dreams and gave him cryptic messages.
Imagine if he was a therapist though. The ability to be on the scene and in a person's head when they're in distress? That power's not lame, it's just limited. That boy could easily grow up to be a one hell of a Manipulative Bastard by taking advantage of people with his power.
Matt's father Maury (and, it is implied, Matt, if he learns to harness it) can induce waking dreams, hallucinations, whatever you want to call them, and Molly accordingly dubs him The Nightmare Man.
In the Doctor Who episode "Amy's Choice", the Doctor, Amy, and Rory are trapped in a dream. The man keeping them there introduces himself thus: "If you're the Time Lord, then you can call me the...Dream Lord." Of course, both the dream and the 'real world' are All Just a Dream, and it was simply a manifestation of the Doctor's darker side, but it looks like this for the first half.
The dark fae Mares from Lost Girl have the power to inflict nightmares on people in order to feed off their fear.
A major arc of the 1966-1971 Gothic soap opera Dark Shadows was The Dream Curse. The witch Angelique, in an effort to return the curse of vampirism to her recently cured lover-turned-nemesis Barnabas Collins, cast a spell which caused various people in Collinsport to have a nightmare. The first person to have the dream would be compelled to tell the next person in the chain about said nightmare. The person told would then have the same dream with an added twist, so on and so on, until the curse reached Barnabas and caused him to once more become a vampire.
The title character of Garth Marenghi'sDarkplace describes himself as this in the opening sequence.
In Changeling: The Lost, all changelings have the ability to willingly enter their own dreams or the dreams of others they have a pact with, perceive the dream with perfect clarity, and even alter its contents. The True Fae also have this quality, and... well, it's not pretty.
Dungeons & Dragons has the arcane spell Dream and its variant Nightmare—first allows to send a message the recipient will remember upon waking, second causes restless sleep and some damage.
In Planescape, the Wall of Color between Deep Ethereal and Border Ethereal is also known (less widely) as the Veil of Sleep: those who instead of passing through the Veil find a way to enter into it visit the dreamscapes of whatever plane it envelops. Not that a lot of people care to bodily wander in strangers' dreams. That's where effects like Dream Travel or Nightmare work. It lies between Prime or other specific plane and protomatter-laden mist of Ethereal plane, that is "what may become". The kicker is that it works both ways — sometimes dreamscapes rupture, spilling contents on Ethereal side where anything the dreamer imagined works like magical illusions. Including a chance to become real, no matter how crazy its properties are.
The quori, nightmare spirits from the Eberron setting's Dream Land can do this. Along with their broader repertoire of Psycic Powers, it's one of their most powerful tools for manipulating mortals.
In Nomine. When humans dream they create dreamscapes in the Marches on the Ethereal Plane. Angels and demons can enter these dreamscapes and affect them (and the human inside them).
Both Wraith The Oblivion and Orpheus had Phantasm, a set of abilities which, among other skills, makes it possible for ghosts to interact with, reshape, or travel through dreams.
The Dreamer, one of the Villains from the "Shattered Timelines" expansion for the co-op card game Sentinels Of The Multiverse, is actually a younger version of one of the hero characters (Visionary) whose psychic powers started going haywire, causing her nightmares to come to life.
Did not turn out well indeed. The people trapped in this world eventually lost their sanity and will attack anyone who approaches them on sight. The only people who managed to stay sane were absorbed into their empathic weapons. On top of all of that, the drop rates are terrible even after being upped, meaning that even normal players can go insane if they do enough Dynamis. (Square apparently takes their tropes seriously.)
Len, the mage-familiar/dream demon of Tsukihime, who is the one who creates the Erotic Dream (any of them) Shiki receives as thanks from Arcueid. In addition, in the sequel Kagetsu Tohya, Len is the one responsible for the endlessly repeating dream Shiki is trapped in.
The Neverwinter Nights 2 expansion Mask of the Betrayer features Gann of Dreams, who can walk in and influence the dreams of others. The player can gain this power themselves later in the game.
Feynriel in Dragon Age II: As a dreamer, or "somniari," he can enter the Fade without Lyrium and exercise a certain degree of control over it — to the point that he can kill people in their dreams. This is an extremely rare power, and makes him an irresistible target for demons looking to posses him. If you let him get possessed, he becomes Freddy Krueger and starts driving people insane. If you encourage him to master his powers, he uses his gifts to help people, at one point making a bunch of would-be rapists kill each other. While they were still awake. From the other side of the continent.
The premise of Dragon Quest VI where you correct problems in the real world and the dream world.
El Goonish Shive had one sorceress who made Ellen and another participant replay lives of their Alternate Universecounterparts in dreams to make them "live through" years of personal experience quickly, and slapped a message of her own on the end.
Danny Phantom had a few characters with these kind of powers. The Fright Knight was, as his name suggested, a medieval warrior who had the power to transport one into a dream-like realm based on their worst fears. There was also Nocturne, the Ghost Of Sleep, who fed on the energy of sleepers. With enough power, he had the ability to control what happened in any dream, including allowing Danny to escape his own. Finally, there was Nightmerica, a female movie monster version of Freddy Krugar that was made real by ghost magic. Though she only played a cameo, if she's indeed like the villain she's based off of (and her name gives any indication to her powers), then Danny and his friends are probably lucky to have only fought her once.
In The Dreamstone, the title Dreamstone protects the Land of Dreams from the nightmares sent by Big Bad Zordrak, while the Dreammaker uses it to send out pleasant dreams to the Noops.
In the Transformers Generation One episode "Nightmare Planet", the Quintessons hook a sleeping Daniel up to a machine that brings the characters and settings from his nightmares into the real world, then set them against the Autobots. The monsters were immune to real-world weapons, but Daniel manages to lucid dream and assist his friends.