Martha Jones's TARDIS key

Martha Jones held possession of a TARDIS key.

History
The Tenth Doctor gave it to her after she became a regular companion of his. (TV: 42)

By welding her key to the Archangel Network, the Doctor enabled Martha to go unnoticed via a perception filter as long as she wore the key around her neck. (TV: The Sound of Drums) This ability allowed her to elude the Toclafane during the Year That Never Was. Ultimately, Martha took off her key when she surrendered to. (TV: Last of the Time Lords)