Excalibur

Excalibur was a legendary sword associated with King Arthur.

Much as accounts differed on whether King Arthur and Camelot only existed in a parallel world, whose history leaked out into the Doctor's universe through a dimensional rift in the form of legends, (TV: Battlefield, PROSE: Legends of Camelot) or were real parts of the history of the England of the Doctor's own universe, (PROSE: One Fateful Knight, The Creation of Camelot) there were many contradictory accounts of Excalibur's existence.

History
The Second Doctor believed that Excalibur was not a magical weapon as later legends suggested, but was merely considered a powerful artefact when King Arthur wielded it because it was made of steel in an age where most of his enemies used bronze weapons. It was suggested that Arthur obtained the sword when "a samite-wearing woman in a pond threw it at him", an event romanticised by the later tales. (PROSE: Heart of TARDIS)

At one point, Merlin, King Arthur's advisor, (PROSE: Excalibur of Mars) who was actually an incarnation of the Doctor lying somewhere in the Seventh Doctor's future, (TV: Battlefield, PROSE: Battlefield, etc.) sent Excalibur to Winifred Bambera and Ancelyn. Bambera then travelled to Mars in order to use the sword to prevent Deimos and Phobos from leaving Mars' orbit. (PROSE: Excalibur of Mars)

Arthur wielded the sword at the Battle of Camlann until his death. Merlin placed it with his body in a dimension-crossing spaceship and hid it at the bottom of Lake Vortigern. (PROSE: One Fateful Knight, TV: Battlefield) In the late 20th century, the Seventh Doctor and Ace discovered Arthur's spaceship and retrieved Excalibur. (TV: Battlefield)

once tried to steal it from the Doctor. (GAME: Destiny of the Doctors)

Despite its lack of actual magical properties, at one point, the Golgotha Project collected the sword as part of an assortment of other "magical" objects. (PROSE: Heart of TARDIS) Excalibur was also once held in the Vault. (AUDIO: Mastermind)

Other references
Ian Chesterton facetiously shouted Excalibur after he pulled a ring like object out of the ground on Aridius. (TV: "The Executioners")

Captain John Hart jokingly suggested Excalibur as a name for Torchwood Cardiff. (TV: Kiss Kiss, Bang Bang)

Behind the scenes

 * The novel Heart of TARDIS lists things considered magical, and the list ends with "...and swords of invincibility lobbed at passing knights by a samite-wearing woman in a pond." It goes on to discuss Excalibur's "invincibility." The quoted section appeared to be a reference to Monty Python and the Holy Grail, particularly the "Dennis" scene.
 * Excalibur was the original name for the science-fiction/crime drama which would become Torchwood until Russell T Davies decided to make a Doctor Who spin-off. John Hart's suggestion of Excalibur as a name for Torchwood Three in Kiss Kiss, Bang Bang is a reference to this.