In several of CLAMP's works, dreamseeing and dreamseers are important elements of the plot and can speak with the dead, especially in X1999, Xxx Holic and Tsubasa Reservoir Chronicle, in the latter two cases allowing people to communicate across dimensions.
In the first known version of "Beauty and the Beast", Beauty has repeated dreams of a handsome young man begging her to save him. Only after she agrees to marry the Beast does she realize that he and the man in her dreams are one and the same.
In Death And Ker, Minako has recurring dream-conversations with several keres and people whose Personas are keres. The ones with Jin and Takaya could simply be normal dreams (considering that both of them are presumably dead), but the ones with Souji, Ryoji, and Ker herself fall under this trope - particularly the one with Souji, as a later chapter confirms.
In the Dangerverse by Anne Walsh aka Whodoyouneedtoknow aka the PAGE, a very long series of Harry Potter fics, the Pack and the Pride can do this. Develops as the stories progress so to do the ways in which various characters are able to meet up.
In With Strings Attached, the Fans (specifically Varx) first contact the four (though we only see his contact with Paul) through dreams, or “hypnogogic telepathic contact.” Most of their subsequent interactions are plain old telepathy. However, much later, after Jeft leaves and the other Fans lose their easy computer access to the four, they manage to contact George and Ringo this way.
The researchers in Dreamscape initially trained psychics to project themselves into others' dreams and help them confront their fears. Then the plot got hijacked by an assassinate-the-President-in-his-sleep scheme.
The Golden Child. The Big Bad communicates with Eddie Murphy's character in a dream. After he wakes up, his attractive female assistant informs him that while what the villain said was actually happening, the parts where she suggested they get together and "let nature take its course" actually was a dream.
In the Chronicles of the Kencyrath Jame and Torisen frequently interact and talk with each other in their dreams. This is especially the case when one of them is knocked out, or someone else interferes with their dreams. Some shanir have the ability to visit the dreams of others.
In Dan Abnett's Warhammer 40000 novel Ravenor Returned, Ravenor contacts members of his team in their dreams to confirm that they want to go on working with him.
In Dan Abnett's Warhammer 40000 novel Brothers of the Snake, the comatose Petrok sends a psychopomp to conduct the dreaming Priad into his own dreams. He warns Priad to flee, because he himself is under dream attack from Dark Eldar.
In Lee Lightner's Warhammer 40000Space Wolf novel Wolf's Honour, Ragnor confides in Gabriella that he think his enemy Madox is in his dreams. Gabriella dismisses it as Bad Dreams; he feels guilty about what went awry in an previous encounter. In reality, he is Dreaming of Things to Come.
In Dan Abnett's Gaunt's Ghosts novel Honour Guard, the wounded Ghosts, left behind, received repeated messages, apparently from the dead, to desert and join the honour guard.
In Only In Death, the dreams plaguing various Ghosts are revealed to be Soric attempting to reach and help them.
In Andre Norton's Sorceress of Witch World, Kaththea's first real communication with Hilarion is in her dreams. (When she had stumbled into his prison, he had tried to take over her mind first, but then, he was desperate.)
In fact, this trope happens fairly often in her works. The Key of the Keplian contains another example.
In Horn Crown the hero dreams of himselfat a long-ago banquet — where a woman talks with him and realizes he's from the future and gives him some aid.
In Robert Jordan's Wheel of Time, Tel'aran'hriod - the world of dreams - is an alternate reality. People can enter it unwittingly in sleep; dreamwalkers can enter it deliberately.
In later books, Elayne invites a device that allows non-dreamwalkers to enter Tel'aran'hriod so dreamwalkers and nondreamwalkers can meet there and exchange information.
In a notable example from Mercedes Lackey's work, Kerowyn, the protagonist of By the Sword, spends ten years having dream-conversations with Eldan before finally learning that they'd actually been communicating telepathically the whole time.
Anne Bishop has used this.
In Terry Pratchett's Equal Rites, Esk dreams about the Things from the Dungeon Dimensions and interacts with them. They even tell her the word "psychosomatic" and assure her she can die because of her dreams.
In Lois McMaster Bujold's The Paladin of Souls, a god appears to Ista in her dreams while looking like his priest, dy Cabon. After the god has finished speaking with her, he leaves and dy Cabon's awareness fills the dream body. Sadly, as soon as Ista figures out that this is a true dream, each the other's, she wakes up. Thus she learns that dy Cabon is alive, but not where he is or whether anyone else has survived.
This is how the cats of StarClan generally communicate with living cats in Warrior Cats, although the cat must believe that StarClan exists for them to be able to do it. Blind Seer Jayfeather can also do this, which is a huge deal because, well, he's still alive. Indeed, he often gets exasperated when the dreams he has frequently do not turn out to be his dreams at all.
Cats in the Place of No Stars can also communicate like this, but it is somewhat vague as to what the rules are for who they can and cannot speak to. The books have also shown that any injuries sustained in The Place of No Stars is inflicted on the cat in the real world too, possibly to the extent where you can be killed in a dream.
In Anne McCaffrey's Talents series, the Mrdini have a limited ability to manipulate human dreams. This works out quite well for first contact betweeen humanity and Mrdini: they're able to communicate through dreams until they learn enough of each other's language to do so verbally.
Angels in The Bible use dreams to communicate with humans: for example, an angel comes to Joseph in a dream to explain to him the circumstances of Mary's immaculate conception, and later returns to warn Joseph that Herod wants to kill the child.
The Five in The Power Of Five books can do this. They all meet in their dreams before any of them meet in real life.
In the Star Trek Novel Verse, the Cardassian Fates communicate like this. Non-corporeal creatures inhabiting a mysterious dimensional plane that intersects with our own, they can telepathically influence mortals. In particular, with individuals of the right genetic makeup (or whose minds have been altered by particular artifacts), they can appear in dreams and hold "conversations" - or alternatively just plant images and desires. In the Terok Nor books, their apparent leader, Oralius, uses it to find the next Astraea so as to keep the Oralian Way religion and the compassionate, noble aspect of Cardassian society alive. Her Evil Counterpart Uramtali uses it to telepathically rape young boys.
In Neil Gaiman's Stardust, Tristan is asked, in his dream, to keep down the noisiness of his dream.
In Patricia A. McKillip's The Book of Atrix Wolfe, the woman in the woods communicates with both Atrix Wolfe and Burne this way. Later, so does Talis.
In the Anita Blake series, Anita starts the process of becoming Jean-Claude's human servant in the first book (Guilty Pleasures), which means he can get in her dreams. At first, he shows up in a coffin overflowing with blood. In later books, this comes up again, and the dreams span from seduction to sex.
Solaris subverts this; while on the space station, the asleep protagonist is seemingly visited by a dead crewmember in his dreams. The visitor states that he's fine with being dead, then warns the protagonist about a secret plot perpetrated by the other people on the space station. It would suggest that there is an afterlife, unusually for Stanislaw Lem's atheistic views (and the atheistic setting.) But it all turns out to be just a dream, born from the protagonist's internal concerns; there's no secret plot at all.
In the story about how the Moonstone was found, Wind appears in Mothflight's dream - the StarClan cats tell her to allow Mothflight back into the Clan, and she agrees.
Leafpool is guided by Feathertail, a StarClan cat, into Willowpaw's dream; they decide how to teach Willowpaw about StarClan, since her own mentor doesn't believe in them.
Jayfeather has the ability to walk in other cats' dreams. He puts it to use occasionally, once by walking in Ashfur's dream to speak privately to him (to persuade him not to reveal a secret), once to talk to Kestrelflight to figure out where there's some herbs (though Kestrelflight thought it was just an ordinary dream), and many times for Dream Spying, listening to conversations between the other medicine cats and their Clan's ancestors.
In Robin McKinley's Sunshine, after the final battle, her grandmother appears in Sunshine's dream to talk things over.
In Palimpsest, people will understand you regardless of what language you speak. Characters run into people they know on the other side frequently.
methought Close at mine ear one call'd me forth to walk With gentle voice, I thought it thine; it said, Why sleepst thou EVE? now is the pleasant time, The cool, the silent, save where silence yields To the night-warbling Bird, that now awake Tunes sweetest his love-labor'd song; now reignes Full Orb'd the Moon, and with more pleasing light Shadowie sets off the face of things; in vain, If none regard; Heav'n wakes with all his eyes, Whom to behold but thee, Natures desire, In whose sight all things joy, with ravishment Attracted by thy beauty still to gaze.
In season eight "The Long Way Home" Buffy has a long talk with Ethan Rayne in her dreams, who gives her valuable clues into who's attacking her, and why.
Restless is an episode of this trope, with the primeval doing the communicating.
In "Graduation Day" after Buffy puts Faith in a coma, then ends up in hospital herself, she has a dream in which Faith appears to reconcile with her, giving a clue on how to defeat the Mayor.
Buffy: (with a half smile) "Is this your mind or mine?" Faith (laugh) "Beats me."
In the Angel spin-off, Faith defeats Angelus by shooting up with a Fantastic Drug and letting him feed on her. They both fall into a coma and end up reliving Angel's life, with both providing appropriate snarks and jibes.
In Heroes, Angela wakes up Sylar through this while they're both comatose.
The Star Trek: The Next Generation "Night Terrors" has the ship trapped in a Negative Space Wedgie, with most of the crew hallucinating and slowly going insane due to lack of REM sleep, while Troi, The Empath, is going just as mad from nightmares about twin moons and voices whispering "eyes in the dark, one moon circling". It turns out there's an alien ship trapped in the same wedgie, and they're using telepathy to try to communicate through the crew's dreams and suggest an escape plan.
Star Trek: Voyager. The Borg Queen communicates with Seven of Nine while she's dreaming in "Dark Frontier" and "Endgame."
Shiki has lots of dreams in Tsukihime but as you get into the farther routes, at one point SHIKI becomes aware he is being watched and starts screaming for Shiki to leave. Throughout lots of the dreams there's also a sort of not-quite communication going on. This is exclusive to these two characters (and maybe Akiha) apparently as the two of them each have one quarter the life of Akiha and therefore are linked, sharing energy, some traits, dreams and mental influence.
Aerith has a conversation with Cloud in his dream in Final Fantasy VII after she's left the party, to tell him about her plan to stop Sephiroth and to reassure him that everything will be okay. Unfortunately, Sephiroth was also in the dream and, after she's left, he pops in to remark to Cloud that that's interesting to know. Cloud immediately wakes up, and the race for the heroes to find her first is on.
Paula uses her psychic abilities to call Ness, and later Jeff in Earthbound.
In the Warden's Keep DLC for Dragon Age: Origins, the mage/Grey Warden Avernus reached out to Levi Dryden in his dreams, encouraging him to explore the haunted fortress that was the resting place of his great-great-grandmother, Sophia Dryden. Fearful of the danger (and rightfully so), Levi contacted the Grey Wardens to help him search the keep for evidence that would
In-game, viewing the dreams of others and actually entering them is stated to be one of the powers granted by Blood Magic.
While they don't exactly have a conversation with it, both the Warden and Alistair wake up from a nightmare involving the Archdemon with the feeling that it actually "saw" them somehow. It's a prelude to an attack by a squad of shrieks.
A Warden's dreams will always be filled with the whispers of the Old Gods after the Joining though he/she can eventually suppress them. The return of the dreams is the first sign that the Taint will soon overwhelm a Warden turning him/her into just another ghoul.
In Dragon Age II Feynriel is a somniari (dreamer), a very rare kind of mage who can manipulate the Fade and enter the dreams of others without resorting to Blood Magic or piles of lyrium. If Hawke helps him survive the demons constantly assaulting him (a dreamer is a very appealing target to demons), Feynriel will go to Tevinter and learn to control his powers. In Act III he saves a young noblewoman from a gang of rapists by entering their dreams and forcing them to kill each other, then comforts the girl by speaking to her in her dreams. The girl becomes smitten with Feynriel and looks forward to when she falls asleep again so she can be with him in her dreams.
Ribbon the telepathic unicorn from My Little Pony could talk to people in their dreams.
The Powerpuff Girls discover they have the power to communicate with each other in their dreams.
In the Season 1 semi-finale of The Venture Brothers, "The Trial of the Monarch", Phantom Limb utilizes this on Doctor Orpheus during the Guild raid on the courtroom, since the typical memory wipe doesn't work on magic users.