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

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