There are a few instances of this in Survival of the Fittest. A notable example is that of Seth Mattlock rescuing Bryan Calvert and Tory Johnson from a player - pulling it off in the true spirit of the trope: just in the nick of time. Amusingly, this is because he waited before pulling off the save, although he was, admittedly, trying to get the best possible shot on the bad guy. It also turns into an unintentional Heroic Sacrifice.
Also in v4, counter-terrorist group STAR show up, take Danya's HQ hostage, and rescue a large number of the students from the island.
When the main characters are cornered in There Will Be Brawl, Captain Falcon comes out of the alley, gives a speech about only he gets to bruise Pit's face, and Falcon Punches Saki and Issac to dust.
Nyx Sparkle is this in Bronyism, who become the first person to successfully retrieve the eponymous page from the bad guys.
Almost every time a new character is introduced in Dead Fantasy, said character is intervening to aid someone else in a fight. In fact, very few fights have had a clear winner, since every time someone's on the verge of defeat, someone else has come to save them. So far, Rikku, Hitomi, Tifa, Rinoa, Cloud, and Ryu Hayabusa have pulled Big Damn Heroes moments.
In Darwin's Soldiers, when terrorists are attacking the control room, Shelton is saved by Cobalt Squad busting through a window.
Subverted in Atop the Fourth Wall at the end of the Mechakara arc. 90's Kid, Ninja-Style Dancer, and Harvey Finevoice show up to save Linkara from Mechakara...but then run away when they realize that their attacks aren't doing anything to him.
Played straight when Pollo returns to blast Lord Vyce and save Linkara's butt.
Whateley Univerese: all over the place in "Ayla and the Birthday Brawl". When Chaka and Dyna-man have been pounded into the pavement by The Lamplighter, Riptide comes to the rescue. When she finally loses, a team of Knights of Purity save Chaka. When Skyhawk ruins the Felonious Four's hostage crisis, Bladedancer and Generator and Shroud leap in to the rescue. When the Felonious Four are about to blow Generator's brains out, she's saved by a hostage kid attending the birthday party. When the supervillains have freed the criminals from the superjail, the four powerhouses of Team Kimba fly in to save the day. When the mercenaries are ready to blow up the restaurant with Phase's friends, a team of power armor guys hired previously by Phase steps in at the right moment.
In entry #52 of Marble Hornets, Masky stops Alex from shooting Jay. He just suddenly appears behind Alex, then tackles him. Then, once Jay's gone, you hear a gunshot. Tim seems to be fine, though...
At the end of The Adventures Of The League of S.T.E.A.M.'s season 2 finale, "Dead End", The rest of the League come to the rescue when Crackitus, the Baron, Katherine and Thaddeus get outnumbered by zombies.
In To Boldly Flee, during the climactic battle, the U.S.S. Exit Strategy is in a pitched firefight with the Co-Dragons of the film, who have a more powerful ship. Just as the Exit Strategy is about to be destroyed, cue Linkara arriving in Comicron-1. He proceeds to put Comicron-1 in front of the heavily damaged ship and draws the enemy fire long enough for the Exit Strategy to finish building a torpedo that can damage the enemy ship.
In Log Entry #26 of Dark Harvest, Alex and Chris have been captured by the Slendy worshipping cult and are going to be executed via Off with His Head!. As the cultists raise their axes, Jeff runs in (they had phoned him and secretly let him know what was going on earlier in the video) and holds the cult at Chekov's Gunpoint until the two are released unharmed.
In the Season Finale, Doom uses Ranger's rune in order to access the operations area of the Vadrigar. Then, he summons each one of the warriors. When they can't defeat the Vadrigar, Doom goes again to said room and summons Gorre, who takes care of the Vadrigar. Cue the Sequel Hook.
In The Livestream Incident from TribeTwelve, The Observer has come to abduct/kill Noah. At the fateful moment, Firebrand steps in and prevents this, in the process revealing that Firebrand is a future incarnation of Noah and that he is now free from the Collective's control.
It's not uncommon in Achievement Hunter's Let's Plays, especially when it involves Team Lads and Team Gents (or the many variations of them) to invoke these, especially if someone gets separated earlier and they come to the rescue. Nine times out of ten, it's usually Gavin.
When the head zombie cornered Scrooge, Louie and Dewey in episode 2 of Ducktalez, Huey showed up and killed the head zombie in one shot with a revolver.