In the climax, this is subverted. The big bad saw the Big Damn Heroes coming and activated his plan as he did. Consequently, they arrive thirty-five minutes too late.
Also subverted with Dr. Manhattan's arrival. Adrian almost destroys him, but he comes back again, crushes his base, and gives an impressive Badass Boast, Adrian merely has to turn on his television to prove he's won, and Jon stands down.
In part 15 of the original series, just as the Wolfriders and Leetah are about to be slaughtered by Guttlekraw's trolls, they are rescued by the Go-Backs and (big spoiler): Rayek. All except One-Eye, who's killed before they arrive. Here's the scene, if all the spoiler tags haven't put you off.
In the climatic battle at the end of the book, Rayek and the Holt Trolls charge in just as the Wolfriders and the Go-Backs were about to be beaten. It is lampshaded with Rayek complaining to Cutter that if he had sent Petalwing the Preserver back to guide them, they could have arrived much sooner.
Y: The Last Man. Natalya and Ciba arrive just in time to stop one of Alter's mooks from kidnapping Beth Junior. Genre Savvy Hero even quips: "Natalya; always with the last minute Han Solo."
He takes it to its logical conclusion in Seven Soldiers, which is just a bunch (conspicuously more than seven) of those moments strung together, from the badass ("Gloriana! There is one!") to the farcical (Bulleteer accidentally running over the Queen of Darkness in a cartoony art style to boot) encompassing romantic, metafictional and slightly creepy along the way even finding room for an ambiguous aversion, with Klarion merrily fulfilling his own personal character arc at the expense of the rest of the "team"
Subverted in his JLA run when The Key devises a plan that actually depends on the Justice League's last minute heroic victory.
Played straight moments later when Green Arrow Jr. shuts him down with a boxing glove arrow to the face.
"And yet, through the smoke, Xavier sensed a coming... of hope... of heroes... of Marvels."
There are some instances in the X-Wing Series comics that count. In Requiem for a Rogue, Wedge is captured by a Sithling who wants to use him as bait for the other Rogues. The other Rogues come on foot to rescue him, and he slips away while the Sithling is directing his forces. Just as three Rogues are about to get killed, Wedge appears with two stolen blasters, shouting "Everybody down!◊"
"Wedge! You're alive!"
"Yeah, and you can keep me that way by moving your behinds!"
In Justice League: Generation Lost #8, the League has been caught breaking into Checkmate headquarters, and are on the verge of being overwhelmed by Checkmate forces. Then the wall blows in, with Captain Atom coming to the rescue:
Ice: Thank God.
Fire: He's not God. He's just our ride.
Justice has a scene where Superman has been dog-piled by four supervillains. Blindsided and beaten on by Bizarro and Solomon Grundy, weakened by Metallo's Kryptonite heart, his powers drained by the Parasite, Superman is helpless. The whole while, he's left wondering why none of the JLA are responding to his distress call, and finally, in a rather chilling scene, Superman cries out for help. Cue Big Damn Hero moment as a lightning bolt flashes across the sky, all of the villains get an "oh shit" look on their faces as a red and gold blur comes down and proceeds to take them apart. Bizarro? Punched into orbit. Metallo? Kryptonite yanked out of his chest. Parasite? Had said Kryptonite jammed in his chest, and since he had stolen Superman's powers and weaknesses, was instantly defeated by it. Solomon Grundy? Beaten unconscious into a crater. In the space of a page, these four powerful villains are handily and decisively dealt with. The blur finally slows down enough to reveal who it is, offering a hand to help Superman up.
Captain Marvel: "It's okay Superman. I got your signal. It's going to be okay."
In Transformers: Timelines "The Stunticon Job", when Sideswipe and Cheetor are cornered by the Stunticons, Optimus Prime disguised as Toxitron comes in, and causes a ramp to crash on the cons.
In Infinite Crisis, The Battle of Metropolis is raging on and it seems that the villains are winning, especially when the Syndicate's flag bearer, Doomsday, charges forward and into the path of Arsenal and Green Arrow. Cue the arrival of Superman and Kal-L, who pummel Doomsday to the ground, followed by the other heroes who had been in the Antarctic with the two Kryptonians.
The final part of Sonic the Hedgehog/Mega Man: Worlds Collide has the original eight Robot Masters jumping in to save Dr. Light, Proto Man and Sonic's friends from the other Robot Masters. In turn, Proto Man, Knuckles and Dr. Light race in to save Sonic and Mega Man.
In Transformers All Hail Megatron, the Autobots were trapped on Cybertron with an army of Insecticons that was busy overruning them. Right up to the point that Omega Supreme arrives and turns the tide singlehandedly. When he was first introduced in one of the Spotlight books prior to that, Optimus said that Omega Supreme existed more or less for that purpose.
It was subverted the first time, however: she bursts through a wall and saves Paperinik from the Evronians... and then tries to kill him because she mistakes him for a Evronian.
In the rebooted Teen Titans #1, Deathstroke is about to points the muzzle of his rifle right in Robin's face and pulls the trigger. Cue Kid Flash appearing out of nowhere, plucking the bullet right out of the air, and quipping, "Bit of overkill, isn't it, using an elephant gun on a helpless teenager?" What follows is a truly epic curbstomping by a superfast yellow and red blur against a one-eyed assassin.
When Jessica Jones has been kidnapped by a group of Hydra agents in The Pulse, a S.H.I.E.L.D. strike team led by Jessica's ex-boyfriend Clay Quartermain rescues her.
In Death Vigil, this is part of Bernie's job. She and Hugin show up in the first issue to rescue Sam from a banshee.
In Kick-Ass, Hit Girl pulls this after she disappears by being shot and falling into the water. The mobsters assume that No One Could Survive That, and assume she's dead. This allows her to sneak up on them just in time to save Kick-Ass from getting killed.