Dream Spying

Dream Spying occurs when a character dreams of something that is currently happening somewhere else. For example, the hero passes out and dreams they're in the war room of an enemy and getting details on their plot.

Mostly seen in fantasy or supernatural stories, and may be explained with a Psychic Link. This is useful for giving the protagonist information they wouldn't have access to any other way. The dreaming character rarely has control over the power, and is often forced to watch scenes they don't want to. May invoke Exact Eavesdropping, although the less understood, the more challenging for the hero. Time constraints are often strict; even if you can rely on the dream spy to find out what the enemy is up to, you will often have to wait for it. Can also shade into Talking in Your Dreams.

Protagonists beware: If the eavesdropped is aware of the spying, they may use that knowledge to set a trap.

Typically ends in a Catapult Nightmare. Compare Dreaming of Things to Come (the vision hasn't happened yet), Dreaming of Times Gone By (the vision is long gone), and Psychic Dreams for Everyone (which is a general rule that fictional dreams tend to be prophetic). Note that this is not about people

Comics

 * Happens a few times in The Sandman and the spin-off Lucifer

Film

 * Dreamscape is about the government setting up a program to train psychics to do this intentionally on behalf of the government, as well as occasionally engage in the odd dream-assassination or two.
 * Happens in Star Trek Nemesis where Shinzon uses a physic to, basically, rape Troy while she's asleep. Of course this comes back to haunt him when she uses this link to locate his cloaked ship.
 * The job description of Extractors in Inception. They infiltrate other peoples dream and then manipulate them to think about their greatest secrets. The secret will then show up somewhere in the dream where the extractor can study it and remember it when he wakes up.

Literature

 * Harry Potter, most frequently in the fifth book. Harry's connection to Lord Voldemort allows him to "see" what the Dark Lord is up to, but it sometimes goes both ways. It is twisted when.
 * Happens almost once a chapter in Percy Jackson and The Olympians-- Percy eavesdrops on villains' conversations pretty much every time he dreams (which is a lot), although the aversion of As You Know means the information he gathers is rather cryptic. Played with in the third book, when
 * The Fellowship of the Ring.
 * While sleeping in Tom Bombadil's house Frodo dreams of Gandalf being rescued from Orthanc. Gandalf later tells him that he had the dream after it had already happened (according to the official chronology, about nine days later).
 * While in the Prancing Pony Inn, Frodo dreams of a horn blowing the same night horns were blown in Crickhollow to warn of the presence of the Black Riders.
 * The Wheel of Time series, in an indirect way. The dream world has none of the people that the physical world does, but inanimate objects are still there so some of the Aes Sedai would read reports from their enemy's desks, though the papers would disappear if it was moved in the real world while they were reading them.
 * In The Black Company, one character gets temporarily dislodged from his body (the enemy wizard was being an asshole) and upon returning discovers, that he can do astral travel whenever asleep. This becomes a major plot point.
 * Happens fairly frequently in the Tawny Man trilogy, as a side effect of Fitz having the Skill. Prince Dutiful also seems to have experienced this, and since Nettle's primary Skill-ability is control over dreams, it's likely that it's happened to her as well.
 * Jane Studdock in CS Lewis's The Hideous Strength discovers she has this ability. It also turns out that one of the bad guys, Prof. Frost, is capable of spying on her dreams.
 * In Blood of the Mantis, Acheous uses this to find the box.
 * In the early chapters of The Pilo Family Circus, Jamie ends up being stalked in his dreams by the Clown Division, who are trying to figure out where he lives. Much to his horror, they succeed. It's also revealed that this is an ability on loan from the Fortune Teller, as Gonko is heard to mutter, "Goddamn, this Dream-Stalking is a con job. How many bags did that skag charge us for this?"
 * Bran from A Song of Ice and Fire has "wolf dreams", where he sees the world through the eyes of his direwolf. This happens to Jon, too.

Live Action TV

 * Happens in Star Trek Enterprise with Trip and T'Pol having some sort of weird link. While she is meditating and Trip is day-dreaming they both meet up in a white blank space. Neither of them consciously wanted to be in each other's thoughts but were due to Vulcan psychic links.
 * Medium is a detective show about a psychic who solves crimes. Her visions range between premonitions (dreaming of murders that haven't happened yet) and this (dreaming of the murderer while he's killing his victim).

Theater

 * Angels in America has the "threshold of revelation," a sequence where Harper turns up in Prior's dream and Prior turns up in Harper's hallucination. They actually have an extensive conversation, but end up revealing details to each other that they normally would have no idea of (Harper knows Prior's dying of AIDS, Prior can tell Harper's husband is gay).

Video Games

 * Not exactly spying, but the Warden and Alistair in Dragon Age: Origins can sometimes glimpse the size and movements of the Darkspawn horde in their dreams.

Web Comics

 * The whole point of Dreamless is that each of the main characters dream the life of the other one while they sleep.