Jack Harkness's TARDIS key

Jack Harkness possessed a TARDIS key.

History
It was given to him by the Ninth Doctor at some point during their travels. Jack used the key to unlock the TARDIS when he discovered it in Archive Six of the Game Station. (TV: Bad Wolf)

By welding his key to the Archangel Network, the Tenth Doctor enabled Jack to go unnoticed via a perception filter as long as he wore the key around his neck. Nevertheless, he failed to elude onboard the Valiant. (TV: The Sound of Drums)