Human Notepad

He Couldn't Find a Pen, but luckily the tattoo parlor was right next door.

Generally a subset of Power Tattoo, which sometimes overlaps with Mark of the Beast and Scannable Man. The Human Notepad is when a character is covered with writing. Perhaps he was the victim of a prank (expect frat boys to pull this one), or perhaps he is making notes on himself as an aid to memory (in which case this is mostly seen on the back of his hand). In Speculative Fiction, it may indicate possession by some powerful foreign entity. In supernatural cases, the text usually covers the whole body, although sometimes just the face or another prominent part is covered, and it appears only when the possessing entity is actively attempting to control or providing power to the marked character.

The significance of this particular form of Power Tattoo is apparently that the controlling force causing the phenomenon is intelligent, as well as alien to the consciousness of the recipient. Thus it is often eldritch or archaic in nature, and as a result completely unintelligible; the further in appearance from any writing system familiar to the viewer, the more inhuman the source will ultimately be.

Examples of Human Notepad include:

Anime And Manga

Many sealing and summoning skills in Naruto incorporate pseudo-text into them, as elements of Instant Runes; they count as this trope when scrawled on the body. In a more straight example, when Sasuke's curse is first sealed by Kakashi, the skill takes the form of a large rune surrounding Sasuke, which is composed entirely of pseudo-text that crawls up his skin and compresses itself to form a barrier around his curse mark.
- A bizarre example are the Toad Scrolls. Naturally born with an elongated body that can unroll like a scroll, secrets and seals can be written on their flesh for safe-keeping and they will then hide themselves within their contractor. Gerotora, the only one seen thus far, contained the 'key' to the Kyuubi's seal.
- Fukasaku became a non-mystical Toad Notepad when Jiraiya burnt a coded message onto his back.
- The Rikudou Sage is the most pronounced example in the series. The patterns on him are seen on others as well, Pain and Madara for example. Then Naruto's Kyuubi Chakra Mode has those markings etched into the aura; they become fully-formed (complete with Badass Longcoat) when Naruto and Kurama fight together.
Miharu suffers from this in Nabari no Ou when the Shinrabanshou sealed inside him is used. This "secret art" is described as being more like an immense library of powerful information, though not in a readable form and thus impossible to control. Inside his mental space, it appears as a stormfront of floating Kanji.
Those Who Hunt Elves.
Kenichi: The Mightiest Disciple has music-obsessed Siegfried writing one of his compositions on his unconscious opponent.
Riza Hawkeye in Fullmetal Alchemist has a tattoo on her back that contains her father's research into flame alchemy, which she shows to Roy Mustang before asking him to destroy it.
- Scar's brother did this to himself in the first anime in an effort to create a true Philosopher's Stone.
A rather disturbing example occurs in Shigofumi. Fumika, the protagonist suffered from a form of child abuse wherein her Mad Artist father would etch his rough drafts into her body with glass pens.

Comic Books

Indirect example: Brian had Dave and Bob tattoo his spellbook on their characters in Knights of the Dinner Table.
Le Scorpion: In The Treasure of the Templars, it is revealed that Rochnan was punished for pretending to convert to Islam by having the 99 names of Allah tattooed over his body.
One of the villains in Matt Fraction's Casanova hid the map to his money in invisible ink on the nubile flesh of a band of pop-singer assassins.

Film

The Pillow Book
The Tattoo in Bulletproof Monk magically transfers to Jade and Car when they take over the magic duties.
Dan, the son of Yellowbeard, has a treasure-map tattooed on his head.
- This was also done in Cutthroat Island, where one-third of a treasure map is tattooed on the scalp of the heroine's father. After he gets killed at the beginning of the movie, she cuts it off of him and takes it with her, as he'd instructed while dying.
In Waterworld, the little girl who serves as the heroine's Morality Pet has a treasure map tattoo that leads to the surviving last dry land on the planet.
Leonard Shelby of Memento uses tattoos as a Note to Self: to make up for his inability to form new memories.
Happens in the film version of Harriet the Spy in the form of "foot tattoos" shared between friends.

Literature

Vinculus in Jonathan Strange & Mr Norrell.
Clive Barker's Book of Blood
The fantasy novel The Warded Man revolves around this—the title character discovered symbols that can kill demons, then trained as a Bare-Fisted Monk and got the appropriate tattoos.
The Death Gate Cycle: Two races of rune-wielding demigods exists: The Sartans, who inscribe Instant Runes by moving their hands and feet - essentially 'dancing' the magic out; and the Patryns who follow this trope. They tattoo magical runes across their entire body (except the head, where it would disturb the process of free thought) and can thus call out their power at will.
In Inkheart, many of the Big Bad's conjured creatures end up with lines of text in awkward places on their skin (like diagonally across their faces) due to a Malfunction Malady of sorts. This carries over to the film version
In Bolesław Leśmian's book version of Sinbad the Sailor, the protagonist's poet uncle laments that his poems always get destroyed one way or another, no matter on what media he records them. Eventually, he tattoos all his poems on his body.
Given the unpredictable nature of his visions Talos will often scrawl portions of prophecy on his warplate, since it may be the only way for him to remember what the heck he just saw. Unfortunately, it makes him distinctly distinguishable as a much coveted prophet of the VIII Legion.

Live Action TV

Toby in the Doctor Who episodes "The Impossible Planet" and "The Satan Pit": Whilst he's controlled by the Beast, Toby's eyes glow red and his body is covered with inscrutable symbols written in what appears to be sharpie.
- Later in "The Impossible Astronaut" and "Day of the Moon", the Doctor's companions write tallies on themselves to remind them of encounters with the silence.
  - For some reason, they like to do it on their faces, where they can't see it until they look in the mirror.
  - And in the beginning of "The Wedding of River Song".
Michael Scofield, of Prison Break, had the details of his escape plan and blueprints of the entire prison tattooed on his body, albeit hidden within geometric shapes and images of angels and demons.
An ep of CSI New York had a vic covered in writing rolled in a carpet; he was a writer killed by the guy he was plagiarising who had a condition that compelled him to write on any surface.
Something to this effect appears to be happening to Max in Dark Angel.
Short, dramatic example: On Lost, Charlie writes "NOT PENNY'S BOAT" on his hand as the room he's in fills with water to tell Desmond.
In season 4 of Heroes the season's Big Bad is a man with telekinesis. He uses his ability to leave messages in ink on his mooks (and form a strangling hand if they anger him)
In Red Dwarf, Rimmer attempts to cheat his way through an exam by covering his entire body (save his hands and face - and buttocks) with written notes. On trying to look at the ones on his forearm, he finds his sweat has smudged them into complete illegibility. This doesn't stop him from wrapping his hand around his forearm and slapping it down on the exam paper, in an insane hope that the words would somehow form themselves. (He then stands up, salutes his examining officer and faints in the middle of the classroom.)
Brimstone : Zeke Stone has the names of the 113 damned souls he's hunting tattooed on his body, 'penned in [the Devil's] native tongue'. When he sends a damned soul back, their name is removed, painfully.
In M*A*S*H, Klinger does this to cheat on a test, requiring him to perform some rather odd gymnastics in order to see the answers.
Angel did this in Angel's dream sequence in "Release". The shaman who removed Angel's soul had writing all over his body and the team had to decipher it.
Buffy the Vampire Slayer. In "Restless" Willow's dream begins with an erotic scene in which she paints Sappho's Hymn to Aphrodite in Greek letters on the back of a naked Tara. And in "Villains" she absorbs several Tome of Eldritch Lore—the text is seen flowing across her skin until its reaches her face and gives Willow Black Eyes of Evil.
Chouriki Sentai Ohranger: Gorou gets the code Red Puncher needs to combine with Ohranger Robo printed on his back by the memory pack containing it when said pack is destroyed.

Newspaper Comics

Jeremy in Zits frequently writes notes all over his body, leading Walt to refer to him as "Our son. The human Post-It note."

Tabletop Games

A few classes in Dungeons & Dragons can scribe magic scrolls onto their skin; the ink or scars remain until read to cast the spell, and then vanish. The 3.5 Edition supplement Complete Arcane also provided rules for wizards to use tattoos as a spell book.
Talislantan Thralls, although this is partly because they'd all look the same otherwise.
Runemasters in the World of Warcraft RPG utilize tend to completely cover themselves with runic tattoos.

Video Games

The Nameless One in Planescape: Torment, put there by a previous incarnation as a Note to Self:.

"No wonder my back hurts; there's a damn novel inscribed there!"

In Rift, the Defiant faction's resident stouts canonically have this as a cultural trait:

"The Bahmi cover themselves in very intricate and magical tattoos called "sefir". The application of sefir is all that remains of the Bahmi's elemental heritage, and they recognize great accomplishments in a Bahmi's life. The tattoos recount the personal histories of each Bahmi, while also providing a sizable source of imbued elemental power."

In The Curse of Monkey Island, one character has the only map to a certain island—tattooed on his back. Yes, you do have to get it off. No, we won't say how. You don't want to know.
In Call of Duty Black Ops, the German who invented the Nova 6 chemical is covered in tattoos of mathematical and scientific formulas.

Web Comics

Mnemosyne in Zeus and Sons has tattoos of things she should try to remember. Including one that urges her to "find a solution in case writing space runs out".

Web Original

This sketch by Derrick Comedy features a man who tattoos himself to remind himself what dicks look like.

Western Animation

Used for humorous effect in Family Guy: When the Griffins go to a prison, they have to write a lengthy contract on a prisoner's buttocks with a shard of glass as he screams in pain. Peter makes a mistake and proceeds to 'erase' it causing the man to collapse, presumably dead.
A squicky variation shows up in Aqua Teen Hunger Force, when Carl and Master Shake anger a forest of talking trees. The trees hang Carl up by his feet and tear off pieces of his skin to write notes on during his "trial".
Mulan writes notes to herself on her arm before heading to the matchmaker, although they are washed off during the scrubdown she is administered by her mother and grandmother making the script non-legible.
In an episode of Futurama, when Fry becomes the emperor of a race of liquid people after accidentally drinking the previous one, he is required at his coronation to recite the oath of office (a Long List of how each emperor dispatched the last) by memory, and gets by by writing it on the inside of his arm.
In an episode of Camp Lazlo, some insects bit Raj in order to make him into a treasure map.
The Simpsons: Homer has written notes on himself in some episodes.
- When he became an ad for blue pants, he ended up writing other ads. It turns out the one about eating Maine potatoes wasn't an ad, but a note to self.
- When Springfield was divided into two phone areas, Homer had the second area code written on himself, as well as a reminder that, out of his two best friends and co-workers, Lenny was the white one and Carl was the black one.
- When Homer entered a Catholic school, he wrote onto his right arm a long word and on his left one "God=Good Devil=Bad".