Excalibur: the legendary sword of King Arthur and the go-to name when you need a Cool Sword. Given its status as a Public Domain Artifact, there are quite a few references to it in various media, not to mention the actual Arthurian literature.
The name Excalibur is derived from from the original Welsh Caledfwlch or Caledvwlchnote pronounced "Caledvulkh", meaning "hard cleft" or "battle breach" depending on the translator. This was rendered as Caliburnus in Latin. Over the centuries the spelling further developed into Escalibor and other variations, finally ending up as Excalibur. Some versions say that Arthur pulled Excalibur from a stone (or anvil) to prove his right to be king. Other versions say that the sword from the stone was not Excalibur, but a different sword, and Arthur instead was given Excalibur by the Lady of the Lake. Modern authors have to usually pick one of these old versions and stick with it. (Although Excalibur found a way to combine the stories). Sometimes modern authors try to distinguish the swords by giving each one a different version of the name "Excalibur", like calling one of them "Caliburn" instead. There are also a few obscure medieval stories where Arthur in fact has another named sword, or a couple of other named swords, besides Excalibur, and so perhaps gives away or lends Excalibur to one of his knights, such as Gawain or Lancelot. Sometimes modern authors use one of these other named swords to distinguish the sword-in-the-stone from the sword-given-by-the-Lady-of-the-Lake.
To avoid confusion when adding examples, please refer to the following pages:
Hellboy as the last living descendent of King Arthur is offered Excalibur so that he can raise an army to fight the Queen of Blood (a.k.a. Nimue).
The Shining Knight in Grant Morrison's Seven Soldiers wields the sword Caliburn Ex Calibur.
In Mage Kevin Matchstick gains the ability to wield the magical baseball bat Excalibur.
In the strip "The Jocks and the Geordies" from The Dandy, the Geordies once decided to search for Excalibur in hopes that with it, they could rally all the great Geordie hard men and batter the hell out of the Jocks. They failed dismally in this hunt, so instead they faked up an Excalibur from tin foil, which did the job just fine.
In Camelot 3000, King Arthur must retrieve Excalibur from the lake, which is now the cooling pond for a nuclear power station.
In Jericho, although it is never once actually described, the sword that Jericho carries is suggested to be this. When Snechta, the High Priestess of the Cult of Chêngrêla, sees the sword, she goes wide-eyed and asks the sword is Caledfwlch, the blade of this world's King Arthur, Aloysius der Lichtbringer. Jericho shrugs and says that, in the Reich whence he comes, they called the sword Kaledfulch, but suggests that the two swords are "likely the same thing". Although, the story of how he came by it is... odd:
I stole it from a museum, but it’s just a superb weapon that laughs at light armor and scoffs at cover. Plus, that museum dressed like a slut and totally had it coming.
It should also be noted that Jericho carries a pair of ammo-less revolvers, which, he notes, were given to him by a lady in a lake. The particular story almost exactly resembles the Lady in the Lake who gave King Arthur Excalibur.
In Quest for Camelot, Excalibur is lost and becomes the MacGuffin as the heroes quest to find it and save the kingdom. Sir Ruper, the Big Bad, seeks the sword so he can magically bind himself to it and become invincible.
In King Arthur (2004), Excalibur / Sword in the Stone is retrieved by young Arthur from his father's grave.
The Last Legion presents Excalibur as a sword first wielded by Julius Caesar which is taken from Rome to Britain by its last emperor, Romulus Augustus. At the end of the film, Romulus (strongly implied to be Uther Pendragon) drives the sword into a stone. The last shot of the film shows the sword in the Present Day, still in the stone and with the original Latin inscription on it covered by moss, save for the letters which spell out 'Excalibur'.
Thomas Malory's Le Morte d'Arthur is widely considered the definitive version of the Arthurian legend.
In The Dresden Files, it's heavily implied - and eventually confirmed in Ghost Story - that Michael Carpenter's sword Amoracchius is Excalibur. He even mentions that, in the past, a wizard (Merlin) held on to it between wielders.
Live Action TV
Merlin, of course. It was created in the episode called "Excalibur", has been thrown into the lake by Merlin, was retrieved by his watery girlfriend, and is currently residing in a large stone. Interestingly, three seasons in and it has still not been named or identified as the legendary Excalibur, nor wielded by Arthur himself.
As of the end of the fourth series, Excalibur has been pulled out of the stone and is in Arthur's possession, but still hasn't been called "Excalibur" yet.
Appears in the Doctor Who story Battlefield, where it is key to resurrecting a King Arthur from an Alternate Universe. Only, Arthur's really long dead...
In the miniseries Merlin, the titular character is given Excalibur by the Lady of the Lake. Merlin gives the sword to Uther Pendragon, but then takes it back when Uther proves not to be worthy of wielding it. He then places it into the care of the Rock of Ages, asking him to only relinquish it to a man with a good heart. A teenaged Arthur later shows up and successfully shifts it from the stone after the Rock witnesses his goodness. After Arthur's death, Merlin returns the sword to the Lady of the Lake.
Crystal Caliburn is named after the sword ("Caliburn"), and it is one of the knightly weapons to be retrieved.
Truth In Television
Truth In Television from which the Excalibur myth was taken: in ancient times, aging celtic heroes threw their inherited, heirloom blades into the swamps to appease the spirits who dwelt there, who presumably kept their weapons safe after they died. They've found tons of swords buried in the former lake around the former isle of Avalon, aka Glastonbury Tor.
The Final Fantasy series has featured Excalibur or variant spellings as a weapon in nearly every game. If there's a sword wielding character, you can bet they'll end up wielding it.
Several games in the series feature a shoddy knock-off Excalipur. Sometimes it's pretty good if you throw it. It's usually found in the hands of Gilgamesh, who has once again mistaken it for the real deal.
Final Fantasy IX has, in addition to the regular old Excalibur an even better Excalibur: Excalibur 2. It's pretty much impossible to get without specifically trying for it.
In Golden Sun: The Lost Age and Dark Dawn, the Excalibur is one one of the strongest swords in both games and obtained by forging an orihalcon with a 5% probability.
Castlevania: Aria of Sorrow had the Excalibur show up as a sword in the stone...amusingly enough, Soma's not the chosen one who can free it, so instead he swings the massive bolder the sword's attach to as well as the sword itself.
In the NES game Day Dreamin Davey, one Medieval stage has Davey overcome all obstacles in order to find the sacred sword Excalibur, only that the "sword" that he has imagined is actually a ruler while he's sitting in the corner in class.
Excalibur appears in Magicka in a similar fasion as it does in Chronicles Of Sorrow; Sure, you can't use it as a sword, but it makes a damn good mace in the mean time.
In Nethack players can recieve Excalibur by dipping a Longsword into a fountain if they are Lawful.
RuneScape has Excalibur as well, at first taking the form of low leveled sword, used to complete the Merlin's Crystal quest. However, by completing the Seer's Village Hard Tasks, the player can have it upgraded to a significantly stronger off-handed version.
Excalibur is available not as a sword, but as a starter warframe.
In the prequel Fate/Zero, Excalibur disintegrates the actual Cthulhu. This is how great Excalibur's power is.
In the webcomic Arthur, King of Time and Space, Uther Pendragon, Arthur's father, allows people to believe his sword is the legendary Excalibur as a PR exercise. This is the sword that ends up in the stone, while the real Excalibur is in the care of the Lady of the Lake.
That is, when it's not a starship, a global corporation, a marshal's badge ... .
In Homestuck, one of Dave's sidequests in Sburb was to find and retrieve Caledfwlch, aka the Legendary Piece of Shit. It was embedded in a stone, and rather than attempting to pull it, he felt he instead had to break it out. Davesprite had already done the same before travelling back in time, and had already alchemized it into the superior Caledscratch, making the Caledfwlch more or less useless... for a while.
In the Global Guardians PBEM Universe, Excalibur is originally wielded by legendary Silver Age hero Excalibur, who eventually hands it down to his son, the Bronze Age hero Pendragon. Pendragon is currently training his own son to take up the heroic mantle when he comes of age.
ThunderCats. In the 1980s Thundercats, Mumm-Ra seeks out a weapon powerful enough to defeat the Sword of Omens. The only weapon without question powerful enough to do the job is Excalibur. Through trickery Mumm-Ra obtains it and defeats the Sword of Omens after the swords fight on their own. It looks like evil wins until Merlin shows up, takes back the sword and kicks Mumm-Ra's ass.