Excalibur

Excalibur was a sword which existed in at least two different realities.

Of Arthur's universe
Excalibur was the sword of King Arthur of England. When he fell at the Battle of Camlann, the Eighth Doctor as 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)

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

Excalibur was once held in the Vault. (AUDIO: Mastermind)

Merlin sent Excalibur to his own universe into the possession of 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)

Of the Doctor's universe
The of the universe that the Doctor called home was said to not be as much magic, but more of a simple tool of innovation. It only appeared to make him invincible because his sword was made out of steel, whilst his enemies wielded those of bronze. It was said that it could have been a mistranslation or joke as well. (PROSE: Heart of TARDIS)

Despite this, the Golgotha Project collected the sword and an assortment of other "magical" objects. It was suggested that Arthur obtained the sword when a samite-wearing woman in a pond threw it at him. (PROSE: Heart of TARDIS)

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

One of the Great Old Ones thought that the sword might be one of six, and may have originally belonged to a race of beings older than the Great Old Ones themselves.

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.