Psychic paper

Psychic paper was a blank, white card that had special properties. When shown to a person, it could usually induce them to see whatever the user wished them to see printed on it.

Users
The Second Doctor was given psychic paper while employed as an agent by the Time Lords' covert organisation, the Celestial Intervention Agency, the CIA having developed the technology. (PDA: World Game)

The Doctor, in his Ninth, Tenth and Eleventh incarnations, habitually carried it, as did Captain Jack Harkness when Rose Tyler and the Doctor first met him in World War II London. Jack said that the paper was a new technology put into use by the Time Agency which had employed him. (DW: The Empty Child)

Properties
Usually, it showed what the holder of the paper wanted the person reading the paper to see (DW: The End of the World), or vice-versa (DW: The Idiot's Lantern). It could even be used to open doors in place of a keycard (DW: Army of Ghosts), and the Doctor once used it in lieu of an Oyster card to pay before boarding a bus in London. (DW: Planet of the Dead)

Apart from its usual function of subterfuge, the paper could receive messages from beings of sufficient psychic ability, such as the Face of Boe and, to the Doctor's initial surprise, Professor River Song. (DW: New Earth and Silence in the Library) The Atraxi also sent the Doctor a message telling him that Prisoner Zero had escaped. (DW: The Eleventh Hour) When used by an individual with psychic powers, the psychic paper would sometimes bounce the individual's powers back on them. For example, when used by Waechter, it allowed him to see his own future. (NSA: The Story of Martha: The Weeping)

Limitations
In the hands of an untrained user the paper was prone to displaying facts they subconsciously wanted the reader to be aware of, often leading to embarrassment. (DW: The Empty Child)

When the intended thought was too unbelievable, the psychic paper would only display squiggly lines. (DW: A Christmas Carol)

The paper does not work on people of genius, such as William Shakespeare. (DW: The Shakespeare Code)

When wet, the psychic paper acted rather differently and was unable to translate the Tenth Doctor's thoughts properly. (NSA: Wetworld)

When a parking inspector signed the Eleventh Doctor's psychic paper, he believed it wouldn't work, until Amy Pond turned it around the other way, causing messages to appear back to front. (NSA: Apollo 23)

Apparently, one could distinguish psychic paper and real paper by looking for 'fractal lines' on the former. However, this has yet to be successfully demonstrated. (DW: A Good Man Goes to War)

Resistance
Not everyone was affected by the psychic paper. People, or groups, known to be immune to its illusions included:
 * Torchwood One personnel, due to psychic training. (DW: Army of Ghosts)
 * William Shakespeare, apparently due to his incredible mind. (DW: The Shakespeare Code)
 * Ernest Shackleton, likely due to the explorer's driven and determined nature. (DWM: The First)
 * Eve (NSA: The Last Dodo)
 * Morret and other veterans of the Telepath Uprising. (NSA: Judgement of the Judoon)
 * GateBots (DWM: Attack of the GateBots!)
 * Rosanna Calvierri (DW: The Vampires of Venice)
 * Frederick Abberline (IDW: Ripper's Curse)

Related items

 * Psychic credit card
 * Suggestibility paper