Donna Noble's TARDIS key

Donna Noble held possession of a TARDIS key.

History
During the Sontaran invasion of Earth, the Tenth Doctor advised that Donna take refuge in the TARDIS amidst the ATMOS gas. It was at this point that he realised he had yet to give her a key and so provided her with one, which he told her was "quite a big moment really." (TV: The Poison Sky)