Fan Works / Badass Bystander

In Boldores and Boomsticks, a group of Grimm attack Golden Valley, and Team RWBY leaps into action, but Ruby is worried that four trainee huntresses will not be enough to defend an unprepared town from the assault. Cue the crowd of Pokémon trainers stepping up to the fray; while they initially leave themselves open to attack, she quickly protects them from ambush and rallies them into a proper defense.
In The Fight we Chose, the Gate opens up at Dallas, Dealey Plaza, unleashing a fantastical pseudo-Roman legion to invade Texas, USA during The '60s. Naturally this being Texas, several hundred armed citizens join the Dallas Police Department in holding off the otherworldly invaders until the National Guard arrives to take back the city.
The Secret Return of Alex Mack: The Macks get kidnapped, but before the kidnappers can drive off, they're confronted by the neighbours. Turns out that traces of GC-161 have been getting into the town water supply, giving superpowers to a significant fraction of the population.

Trade Us for the World: During the Chicka-lisk's attack, several of the residents of Wartwood step up to defend their homes, much to Marcy's astonishment.

In Fox Rain, this is how Lila meets Master Fu: as she was going back home after her original Akumatization she spotted four delinquents her age trying to mug Master Fu and scared them off by pretending Ladybug was about to jump them-and was actually ready to physically beat them up if that failed (and hopeful it happened and gave her the chance to vent some of her anger on them). She then escorted the old man to his home.
In Which the Fox Flees: After getting tied up by Queen Fashion, Ladybug finds Lila freeing her. It's stated that Lila started showing up at later akuma fights.
LadyBugOut: The Recursive Fic Rose's Stand centers on Rose using the self-defense tips she got from the titular blog to defend herself against an akuma, knocking them out cold before Ladybug gets there.
Nymph and the Corrupted Miraculous: Several ordinary Parisian citizens help out the heroes during the Final Battle by arming themselves with improvised weapons to tackle the Null Warriors.
Recommencer: When Marinette's first day at her new school is disrupted by an akuma attack, Chat Noir proves less than useful against Restarter, as he's too busy harassing Ladybug. When Ashli gets involved, Chat Noir complains about her involvement while simultaneously proving that Ladybug needs the extra help by refusing to cooperate unless she 'admits that she loves him'. This forces Felix to step in as well, and leads to Chat Noir being Shamed by a Mob for his unheroic behavior.

Izuku in It Takes a Child to Teach a Village stops a robbery by yelling at the would-be robber about an English test he failed. Turns out, the criminal was one of his students.

In Chrysalis Visits The Hague, the one guy who manages to overpower Chrysalis by activating her magic suppressor as she tries to escape custody (she was on her way to stand trial before the International Criminal Court) is some nameless USAF pilot.

In Chapter 30 of Pokémon Reset Bloodlines, Ash is confronted by two of the Eevee Brothers, who are worried that, having seen a recent tournament where Ash won using Pikachu, their little brother will try to emulate him and fail (context: trying to win his battles using unevolved Pokémon instead of evolved ones), and decide that the only way to avoid that is beating him in a battle. However, a woman steps up saying that if they're going to challenge Ash together, she will team up with him. Said woman is Elite Four member Karen, thus Curb-Stomp Battle ensues.

In The Keys Stand Alone: The Soft World, on the Flying Island of Tipaan, a disguised George and Ringo are jamming along with the Six Step Combo (and having a damn good time doing so). Things abruptly stop when one of the "Punk Cowboys" stands up and accuses the two of being members of the Awesome Foursome. When George and Ringo just sit there looking incredibly guilty and with no clue as to how to plausibly deny the accusation, the three Punk Cowboys get up on stage to unmask them. The Six Steppers, all civilians, promptly beat the crap out of them. Ranala, the group's drummer, does for one of the Punk Cowboys all by herself, beating him over the head with her drum until he falls over. Turns out the Six Steppers knew they were outworlders in disguise but didn't care; indeed, they had good reasons for coming so swiftly and decisively to Ringo and George's aid.

Chasing Dragons: At one point, an Assassination Attempt is made on Robert's wife Serina, his former paramours, and their children. Their bodyguards do their jobs and protect them, but they hardly have to do anything, as the crowds of civilians surrounding them surge forward and subdue the assassins.

We Can Be Heroes: An angry mob of Golden Acres' gun-toting citizens helps drive the Blackthorn Pirates away, then assist Lapis, Cloud Agate and Black Spinel in dealing with the Corrupted Ruby they left behind.

Two examples in My Ridonculous Race. The 1st is Anjel Williams, who offers to drive Jordan and Fabian throughout London when they blow all their money on a hospital visit. 2nd are the Gym Rats Gabriella and Nekota, an already eliminated team who not only find one racer's stolen passport but also offer to drive their team around Washington DC.

Badass Bystander / Fan Works