Finders Rulers

""Listen. Strange women lying in ponds distributing swords is no basis for a system of government. Supreme executive power derives from a mandate from the masses, not from some farcical aquatic ceremony. You can't expect to wield supreme executive power just 'cause some watery tart threw a sword at you. I mean, if I went 'round, saying I was an emperor, just because some moistened bint had lobbed a scimitar at me, they'd put me away!""

- Dennis the Peasant, Monty Python and The Holy Grail

In some forms of government, power derives from the mandate of the people. In others, it's derived from Royal Blood. In yet others, it's derived from killing the previous guy or being the baddest mutha around.

Then there are governments where it's decided by who holds the MacGuffin. In some instances, it isn't even a Only the Chosen May Wield item but something anyone can pick up.

Anime and Manga

 * In Nichijou, the two wooden cubes Mio wears in her hair is shown to be this, though it may or may not have been All Just a Dream

Comic Books

 * In King Ottokar's Sceptre, according to tradition the rightful king of Syldavia must not lose possession of the sceptre of King Ottokar IV.
 * Although there are no indications that having the sceptre would make you ruler, just that the king has to keep the sceptre to keep the throne.
 * The Golden Helmet of Harold Hardrotta in the Disney Ducks Comic Universe confers the rulership of the entire continent of North America to whoever has it. It crops up at the worst times and in the worst hands. (Until a later comic had even more ancient pieces show up!)

Film

 * In The Beginning - Londo tells a little boy that the person possessing the Imperial seal is the emperor. Of course, Londo is quite drunk at the time.
 * In Little Nicky, rulership of Hell apparently belongs to whomever is sitting on the throne.

Literature

 * Council Wars - Membership in the council is contingent on possessing a key.
 * King Arthur - The original Sword in the Stone is introduced this way, and some adaptations run with it.
 * Note that the original legend actually subverts this; nobody cares when Arthur loses the sword and switches to Excalibur.
 * Although in that case, it had less to do with the sword itself, and more with the fact that Merlin said that the person who could take the sword would be the rightful king (which was just a flashy way of introducing Arthur, who was rightful king by birth).
 * Discworld, unsurprisingly.
 * Parodied in a footnote in one book where the person that found the magic sword consisting of two bits of wood nailed together is acclaimed king, because they also appear to have found several thousand heavily armed men along with it.
 * In The Queen's Thief, the MacGuffin is a stone carved with runes, said to have been dipped in the water of immortality. Whoever has it has claim to the throne of Eddis,.
 * In Romance of the Three Kingdoms, Sun Jian finds the Imperial Seal in a well during the sacking of Luoyang. This gives him the "Mandate from Heaven" - in other words, the divine right to rule over China. However, he is killed shortly after leaving Luoyang, and the seal falls into the hands of Cao Cao.
 * In "Queen Zixi of Ix" by L. Frank Baum, the king dies without an heir, and the rules state that the crown goes to the forty-seventh person to walk through the city gates the next morning. Naturally, the story follows the ordinary boy who is the forty-seventh person that day.
 * In L Sprague De Camp's Reluctant King series, the kingdom of Xylar chooses its next king by throwing the head of the previous king into a crowd—the catcher gets the throne. The catch is that in five years, the process is repeated... which is why Jorian, who had no idea about all this, is very much the titular Reluctant King, and spends the trilogy running away from Xylarians who want to drag him back so they can perform the ceremony.

Live Action TV

 * Babylon 5 - The Drazi factions are led by individuals with the leader sashes, making anyone wearing the sash the leader.
 * In the episode Coming Of The Shadows, a Centauri noble mistakenly plans to use a lost piece of crown jewelry to lay claim to the throne. Londo refuses part in it, explaining why the idea won't work.
 * There was some explanation, since the piece was a symbol of his own house more than of the Emperor. He is still heavily mocked by his allies for his stupid plan.

Mythology

 * The legend of Odysseus / Ulysses ends on something of this note, with Queen Penelope announcing that whoever can thread her husband's legendary bow, which only he can use, and successfully use it to shoot an arrow through a dozen axe-heads will be named King of Ithaca. Of course, what no one knows is that Odysseus himself, who has finally managed to reach his home after a journey of ten years, is secretly one of the contestants...

Tabletop Games

 * In Exalted, one of the greatest and wealthiest nations of the South, the city-state of Paragon that has over a million people (not all of them in the city, some are just farmers) is ruled by the man who found an artifact of great power from the First Age. Additionally, there is a second part of that artifact that would allow whoever finds it to share that rule and impose his own will on the populace. Needless to say, it's still lost and it makes for a fine story hook/plot Mac Guffin

Video Games

 * Little King's Story begins with the player character finding a crown and putting it on, proving himself to be the destined ruler.
 * Hilariously used in Knights of the Old Republic. The Mandalorians had a special helmet that was the symbol of leadership. Revan permanently shattered them as a united force by beating the current leader and stealing the helmet (he gave it to a Mandolorian friend to rebuild them years later).

Web Comics

 * In How I Killed Your Master, whoever possesses the governor's imperial seal can issue his orders, making its bearer the effective ruler of the province. Subverted when Wong finds it by accident and asks if this makes him the governor. Fang Lin assures him that if he tries to use it, the factions hunting for it will just kill him and take it.