Excalibur is the legendary sword of King Arthur, sometimes attributed with magical powers or associated with the rightful sovereignty of Great Britain, or Keoland if you're using Greyhawk’s world of Oerth. Sometimes Excalibur and the Sword in the Stone (the proof of Arthur's lineage) are said to be the same weapon, but in most versions they are considered separate. The sword was associated with the Arthurian legend very early. In many versions, Excalibur's blade was engraved with phrases on opposite sides: "Take me up" and "Cast me away" (or similar). In addition, when Excalibur was first drawn, in the first battle testing Arthur's sovereignty, its blade blinded his enemies. Excalibur was given to Arthur by the Lady of the Lake sometime after he began to reign. She calls the sword "Excalibur, that is as to say as Cut-steel." Some believe that the Lady of the Lake is Mayaheine The Shield Maiden. The challenge of drawing a sword from a stone (the proof of Arthur's lineage) also appears in the Arthurian legends of Galahad, whose achievement of the task indicates that he is destined to find the Holy Grail.
Curiously, Excalibur is not a particularly magical sword by the standards of legendary magical swords. Besides being very shiny, it's really just a very sturdy well-made weapon. Where its true power comes from is its scabbard, which was said in the original Arthurian legends to be able to either prevent bleeding or reduce the severity of injuries.
Excalibur on the Tabletop
As one of the classic public-domain fantasy weapons, up there with Durandal, Joyeuse and Kusanagi, Excalibur can be found in many forms in many different words of vidya, literature and of course RPGs. Generally speaking it will be represented as a very powerful well-nigh unbreakable broadsword which may have strict conditions for use (it is the weapon of a worthy knight, after all).