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
Both the Doctor, in his ninth and tenth 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).

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)

Members of the Torchwood Institute received basic psychic training and knew enough not to be fooled by the paper. However, their doors were not so knowledgeable. (DW: Army of Ghosts)

Individuals with powerfully creative minds also seemed to be immune to the effect of the paper, although the Doctor appeared to believe this only occurred in truly exceptional cases. (DW: The Shakespeare Code)

Apart from its usual function of subterfuge, the paper could receive messages from the Face of Boe, to the Doctor's surprise (DW: New Earth).