Everyone Join the Party

Random people reinforcing the protagonist(s) or the opponents for the final battle, for no apparent reason other than to increase the magnitude of the fight. A less predictable version of The Cavalry or just about any Deus Ex Machina.

If all the people are called rather than showing up for no reason, it becomes Gondor Calls for Aid.

Anime and Manga

 * The Whitebeard War was heading this way in One Piece. Every single Marine who ever appeared in the comic was there, plus a ton more. A bunch of New World pirate captains. The surviving male members of Baroque Works. Buggy the Clown. A small army of escaped prisoners and transvestites. Blackbeard and crew. However, it wasn't until  showed up that this really proved to be the case.
 * The end of the second season of Vandread featured lots of people coming to Nirvana's aid against . Although this wasn't really expected, it has been justified by all of them coming from planets where Nirvana's crew did something good or otherwise impressed the locals over the course of the two seasons.

Film

 * The last scene in Monty Python and the Holy Grail, in which King Arthur's small band of knights suddenly becomes a huge army.
 * Subverted in The Matrix Revolutions, wherein the 9 BILLION copies Agent Smith made of himself just stand around and watch Smith 1.0 and Neo fight.
 * In Casino Royale 1967, everyone from the British army to Cowboys and Indians show up for the last battle. There is no reason for this. (Nor, really, anything else in the film).
 * Similarly, the brawl at the end of Blazing Saddles is between all the bad guys and all the townsfolk and spills out into adjacent studios.
 * The Blues Brothers, Inverted. While the police have been chasing Jake and Elwood for the whole film, at the climax, hundreds of police men, SWAT team, soldiers, tanks, and helicopters show up out of nowhere in downtown Chicago to apprehend two unarmed men.

Live Action TV
""My goodness me, it's the Duke of Kent to the rescue...""
 * Parodied in the Monty Python's Flying Circus sketch "The Pantomime Horse is a Secret Agent Film."


 * In Glee almost everyone in McKinley joins the glee club right before a competition so they have enough members to compete. In Season one, Jacob Ben Israel briefly joins to replace Finn, but he doesn't actually compete. In Season two, Lauren joins at Puck's persuasion. And in Season three,

Literature

 * Harry Potter and The Deathly Hallows - everybody and their dog shows up for the Battle of Hogwarts.
 * Possibly overlaps into Gondor Calls for Aid, since a lot of the people who showed up were called upon to be reinforcements.
 * Diana Wynne Jones actually does this a lot in many of her books; Howl's Moving Castle is probably the best known.
 * The Battle of Five Armies in The Hobbit.

Video Games

 * In Final Fantasy IV, the entire inactive party prays to revive
 * Earthbound while fighting the Big Bad, the only way to beat him is to use the up until then useless skill "Pray," which causes all the NPCs in the world, plus the player, to pray together, which destroys the Big Bad.
 * The citizens of Station Square did this in Sonic Adventure. Though how they were cheering, let alone alive, after the city had been destroyed is beyond anyone.
 * Played literally in Castle Crashers where
 * Neverwinter Nights 2: Mask of the Betrayer has the final confrontation taking place in your own mind as it is being devoured by the Spirit-Eater curse, as you fight you call your allies and friends (some of which are dead) to assist you.
 * Similarly, Jade Empire won't let your party members join you in the final showdown, but they (those who remained loyal anyway) will help you take out the Big Bad 's minions.
 * In the PC fantasy kingdom sim Majesty, you (as the sovereign) are not allowed to have certain groups, usually of warring religious factions, in your kingdom at the same time. However, in the final quest, as your kingdom's map gets filled out by exploring parties, they stumble across hidden pockets of every conceivable group of adventurers -- all of whom promptly join in aiding you in the final battles against every villain you've encountered up to this point in the game. All the heroes versus all the villains, including the ones they already killed.
 * At one point in Disgaea 3 a show of support from Mao's friends goes overboard into silly when every minor opponent up to that point shows up to lend their support.
 * When you're facing down Feinne the World Eater in Soul Nomad and The World Eaters for the second time, after she's annihilated the town everyone laments how you cannot hope to defeat her power. Cue textbook Big Damn Heroes moment as the knights of Raide and Christophe's mercenaries show up to lend their aid.
 * When you're facing down Feinne the World Eater in Soul Nomad and The World Eaters for the second time, after she's annihilated the town everyone laments how you cannot hope to defeat her power. Cue textbook Big Damn Heroes moment as the knights of Raide and Christophe's mercenaries show up to lend their aid.

Web Original

 * Chapter 10 of Red vs. Blue: Revelation has just about every main character from the Red and Blue teams who had been split from the party return for an epic confrontation with yet another returning character,.
 * And she then proceeds to lay an utter asskicking against every one of them.

Western Animation

 * Avatar: The Last Airbender had pretty much every character with a name show up to fight on the Day of Black Sun. Except Bumi. He was doing other things...
 * That's more a case of Gondor Calls for Aid. Now, when the Gang (not Gaang, as the Avatar himself was elsewhere) shows up at Ba Sing Se looking for Iroh, they not only find him, they also find the rest of the White Lotus Society... Which consists of all the great masters of the world that aren't directly loyal to the Fire Lord. Cue curbstompbattle against the occupying Fire Nation forces.
 * X-Men: Evolution had an epilogue that featured practically every character from the series who could conceivably join the X-men.
 * Arguably, all of Justice League Unlimited was this, being a show featuring dozens of superheroes as a sequel to a show featuring seven.