Astral Projection

Astral Projection is a magic/mystic practice which allows a character to detach either his soul or his conscious mind (the distinction is important) from his body and let it wander freely, either invisibly in the material plane or out in the Spirit World.

As, essentially, a living ghost, the character can go just about anywhere, see and hear anything, and potentially engage in Invisible Jerkass behaviour, depending on whether he can move objects telekinetically. Also like ghosts, characters who are Astral Projecting may use Demonic Possession to take over host bodies, though this doesn't necessarily allow them to pry into their mind. Most victims usually have no idea what happened afterward.

Some darker uses of this practice will forcibly evict the Astral Self from its host body. If the spirit doesn't know their body is still alive, they may even confuse themselves for an actual ghost.

Of course, it's entirely possible for an Astral Projector to misuse an Ancient Artifact that forces him into an Astral Projection without knowing how to undo the state or return to his body. Or worse, end up in a Freaky Friday situation! On the upside, being Made of Air makes him indestructible to everything except angry ghosts, exorcisms and irate housewives with vacuum cleaners... Of course, that's little help while his body is in a Convenient Coma and extremely vulnerable to attack or neglect (in many cases the body will die if the soul is separated from it for too long). The other vulnerability is that, also as with a ghost, his soul/mind might be unable to find its way back. This is especially risky if the Astral Projector loses the "tether" that connects him back to his body.

Compare Animal Eye Spy, where the character can see other locations through an animal's eyes.

Examples of Astral Projection include:

Anime and Manga

Only the most powerful Newtypes in the various incarnations Gundam have this ability. Examples include series protagonist Camille Vidan and main villains Paptimus Scirocco and Haman Kahn of Zeta Gundam, Judau Ashita in Double Zeta, and Amuro Rei by the time of Char's Counterattack.
Something like this happens in the Pokémon episode where Ash visits the Pokèmon tower in hopes of catching a ghost type Pokèmon that can help him beat Sabrina. While trying to capture one, he and Pikachu get knocked out by a falling chandelier and Haunter takes the oppurtunity to pull their spirits out of their bodies. Ash discovers that the ghost Pokèmon just wanted some friends to play with, but tells them that he can't stay with them because he has to continue his quest to become a Pokèmon master. The ghost Pokèmon are disappointed by this, but they allow Ash and Pikachu to return to their bodies.
Production I.G.'s Ghost Hound features a group of teenage boys who each gained the ability to enter the "Unseen World" after a traumatizing experience they went through in their own childhood. For the former half of the series, they use it to investigate their own past trauma.
The bedridden Grove in Vampire Hunter D has the power of astral projection, his soul becoming a powerful entity, though doing so drains his life.
Schierke of Berserk uses this to communicate with the elemental spirits that power her magic and occasionally to go into Guts' mind to snap him out of the influence of his Super-Powered Evil Side.
In One Piece, Perona uses this to counter Ussop's immunity to her negative hollows.
- During the two-year Time Skip in One Piece, this ability is developed by Brook, apparently as an extension of his Undead Fruit powers. Guess what was the first thing he did with it?
In the Ranma ½ manga an old man has this ability and uses it to enter and alter Ranma's dreams to date his female form.
In the first movie of Kara no Kyoukai, Kirie Fujou, though bedridden, can project a double of herself on top of the Fujou Buildings, which her family used to own.
The Silver Tribe of Heroic Age frequently does this, and are capable of it across the vastness of the galaxy. Dhienalia, the human princess, is also capable of it, and makes use of it for long-distance communication, or guiding the ship through uncharted space. It's apparently pretty well-known, as no one shows the slightest surprise when someone does it.
Yurie, the title character of Kamichu!, discovers that this is one of her powers by accident - and her spirit familiars are as surprised (and more worried) by this as she is, hinting that astral projection is not one of the standard godly powers in that setting. In a later episode, she does this deliberately to visit the Yamato.

Comic Books

Johann Kraus in Hellboy was a spiritualist whose body was killed while during a seance. His soul had nowhere to return to, so he has to live in a containment suit to avoid completely evaporating.
Being the world's most powerful telepath, Charles Xavier does this frequently.
- Despite not having any Psychic Powers of his own, Xavier's sometimes-ally, sometimes-enemy Magneto can do this as well.
As the picture indicates, Doctor Strange has this down to an art form.

Fan Works

In Alternate Universe Fic, Astral Journey: It's Complicated, Emma develops this after a freak accident at a football (soccer) game with Geri. This causes a lot of painful seizures, trying to save Melanie, and meeting a Cardinal named Nicholas, who looks familiar. Brandy, Mariah, and Celine also seeks to help them out.

Film

The thematic foundation of Insidious.
After getting caught by the police, the Big Bad of 976-EVIL uses his One Phone Call and receives this power from the title phone number to continue his wrongdoings.

Live-Action TV

Prue from Charmed develops this as a power in the second season although it's treated more like cloning.
"Weird Al" Yankovic did this once on The Weird Al Show. Once he achieved it, all he did was go watch TV for a while before returning to his body.
Buffy the Vampire Slayer, "Nightmares". A twelve-year-old boy in a coma manages to do this (thanks to the Hellmouth's all-purpose magic-boost), but causes everyone's nightmares to come to life. Buffy has to help him defeat his own nightmare - the guy who put him in the hospital - for him to wake up.

Buffy: Could I be seeing Billy's asteroid body?
Giles: Astral body.

"I'll be Watching You", the first episode of Ghosts, a 6-episode BBC horror series from 1995: The protagonist is a gangster on a long prison sentence. He uses astral projection to spy on and kill his wife and brother (who were having an affair) and an old associate. While he's out projecting, a prison guard finds his apparently lifeless body. The resuscitation attempts cause his projected self to lose consciousness and he wakes up in a coffin, buried next to his wife and brother.

Literature

Discworld: Esmeralda Weatherwax can move her consciousness away from her body and share an experience with a target, or several as in Lords and Ladies when she possessed a swarm of bees. In A Hat Full of Sky, Tiffany Aching learns a similar trick but remains disembodied.
Is a central theme in James Herbert's Nobody True.
The telepathically-gifted on Darkover do this a lot; there's a psychic plane called the Overworld where they do various kinds of work (and occasionally engage in mortal combat...)
Seems to be John Carter of Mars's primary method of traveling to Barsoom.

Oral Tradition, Folklore, Myths and Legends

Older Than Print: Iron Crutch Li, one of the Chinese Eight Immortals, was once a handsome man with the power of astral projection. He told his apprentice to wait seven days before cremating his body, but when the apprentice had to go visit his dying mother he had to cremate his master's body early, leaving his master with no choice but to enter the body of a recently deceased old cripple.

Tabletop Games

Dungeons & Dragons has spells that allowed Astral Travel, usually with a "silver cord" of infinite length that connected the wandering soul to its body; it made the soul virtually indestructible unless the cord was broken, which only very few beings or objects could do.
Astral travel exists in Shadowrun, but there's also "decking," when a character with the proper cyber implants sends his consciousness onto the Internet.
This is pretty common in the Old World of Darkness, just about every gameline has one version of this.
RuneQuest allows players to become shamans, whose souls travel in the spirit world to commune with greater spirits and capture lesser spirits for their magic or other powers. Usually an ally spirit called a Fetch guards the shaman's body. At least as of Mongoose's RQII (RQ5?) advanced shamans can carry other characters' spirits along with them, either voluntarily to journey together or forcibly to engage in spirit combat.

Toys

The Mask of Spirit allows the wearer this power in Bionicle.

Video Games

In World of Warcraft, when players die, they control their characters' spirits, roaming the world from a spirit healer point looking for their body to revive.
In the video game Prey, Tommy's spirit can be projected from his body. Players can use the ability to pass through forcefields, dangerous obstacles, and attack enemies.
- Also, when the player dies, they can shoot down spirits and regain health in the Spirit World to return.
In Bioshock 2, the Scout plasmid allows Delta to astrally project himself, as well as cast plasmids and, with some upgrades, hack machines. The plasmid automatically cancels if his body is harmed, though.
One of the psychic toys from Sam & Max: The Devil's Playhouse is the "Astral Projector". A reel projector that allows Sam and Max to spiritually enter the bodies of their respective Grandparents. As well as allowing Sam to posses any cloned body.
True to his roots, Doctor Strange uses this at the beginning of his Level 3 super in Ultimate Marvel Vs Capcom 3.
In the ZX Spectrum games Avalon and Dragontorc, the protagonist is an astral projection.

Web Comics

In Gunnerkrigg Court going out-of-body allows to see other Etheric entities and magic in colour, over grey shapes of the material world, and even communicate "invisibly"—unless there's another observer in the same state, of course. Strong reactions translate to the body—Antimony and George during the memory-dump didn't move, but wept. Later when Annie saw Lindsay and Bud kissing and when Red was surprised by Annie's trick this reflected in appropriate grimaces on their bodies. At least Gillitie Wood creatures turned humans can leave their bodies "on autopilot" speed-type the lesson in the real world while they fly around and are distracted by something more interesting ("We's don't need our minds to learn dis junk!") and create illusions from memory if they concentrate a little.

Web Original

In the Chakona Space stories Skunktaurs of House Blackpaw have this ability, as do Chakats sired by them and some other individuals of different species.

Western Animation

The sheep talisman in Jackie Chan Adventures does this. Lead to a Crowning Moment of Funny when someone tried to use it in battle.
In Avatar: The Last Airbender, Aang can astral project into the spirit world (or accidentally into the material one). It's suggested normal people might have this happen to them on rare occasions.
Doctor Orpheus in The Venture Bros can do this, usually for the sake of communicating with Jefferson and/or The Alchemist.
- Unfortunately, he cannot double project, which leads to some difficulty when talking to them both at the same time.