Excalibur

Of Arthur's Univurse
Excalibur was the sword of King Arthur of England. When he fell in battle, Merlin placed it with his body in a dimension-crossing spaceship and hid it at the bottom of Lake Vortigern. In the late 20th century, the Seventh Doctor and Ace discovered Arthur's spaceship and retrieved Excalibur. (DW: Battlefield)

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

Of the Doctor Who universe
The of the DWU 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 possessed those of bronze. It was said that it could have been a mistranslation or joke as well.

Despite this, The Golgotha Project collected the sword along with 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.

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