There's a much bigger one in the Vampires storyline, when Riff, Bun-bun, Aylee and Kiki head to save Torg and ZoŽ from vampires. Delays caused by Bun-bun and the others cause them to arrive in the nick of time too, though they had several days to start with. Played only partly straight in the end, as Torg and ZoŽ end up killing the most important vampires themselves, albeit only enabled to do so by the help.
Bozzok: Damn it!!! How do the good guys keep DOING that????
Belkar: Trade secret.
Belkar has one moment when he is cured from the curse that left him half-comatose for several weeks of comics.
Belkar: Back me up and I'll cut a path through these guys. Wait, did I say "path"? I mean a five-lane blacktop highway with a two-lane service road - and I'm packin' a fist full of tokens and a radar detector.
Invoked in Buttlord GT, where Buttlord gets to the battle on time, but refuses to interfere until his allies are half-dead.
Just when it looked like Sixx was lost to Butterfly and Trina, Ginger and Laura break into subspace, which should have been impossible for the latter. Ginger takes out Trina, and Laura gives Sixx a Cooldown Hug, giving her the strength to withstand Butterfly's assault.
Several of these in The Water Phoenix King have been subverted or ambiguous (everyone chasing around in the tunnels in Chapter 1, the anticlimactic fight in the clearing in Chapter 2) but when Gilgam and the Commander finally arrive at the inn just in time to give the magic-users cover enough to repair their defenses at the conclusion of "Pride of the North" it's classic, and Book 4 takes it up to 11 when Anthem and Vish are the only ones left to go try to rescue all the real warriors from the Temple...CMOA, bringing a lot of themes full-circle there.
Just when Rapdevil is about to kill Prince, Kang Horyang appears and beats the shit out of the Da'an Guard and RD.
Yo-Jin-Bo has its own moment of the heroes saving the other half of the heroes. Sayori trips and falls as she and Muneshige are running to first escape the castle. The ninjas have her surrounded and are about to kill her. Enter five wise-cracking heroes. Those poor ninjas never regain their competence.
In the "Armadeaddon" story-arc in Penny Arcade, it looks like Gabe and Tycho are about to be overrun by the zombie horde when they are saved by Fruit Fucker! Fruit Fucker: Make Fruit Your Bitch. And stopping a zombie apocalypse is part of its programming—Code Omega.
Fruit Fucker has joined the party!
In All Manner Of Bad the hideout was attacked while half the group was away retrieving family and loved ones. One of those characters made an end of the world plan with his wife that included zombies. Upon their return he and his wife open fire on the huge horde of zombies attacking their hideout (and former workplace). Partially subverted in the last panel as their self-proclaimed bad assitude is wittily rebutted.
In Cat Nine, Keith saved Myan the first time they met. 
Just as Susan and Anna from Sire are about to be preyed upon by a bunch of London thugs, Emile, the Javert child arrives and administers justice.
A small one in Dreamkeepers Prelude: To condense what leads to it, a drunken, enraged Grunn (the abusive owner of the orphange), after losing most if not ALL of his fishing boats, finally gets so angry that he just snaps and starts throwing the orphans several meters out to sea so that they'd be forced to bring them back. (Admittedly, he had few choices left to get his boats back, his only viable means of making money was selling the catch brought in by those boats, and he had just been repeatedly hit in the face with an oar...) Vi, who somehow avoided Grunn, swims all by herself out to the other orphans (Who are just hanging out on an overturned boat, apparently forgetting that Grunn is waiting for them on land...), putting herself in danger. Why would she do something so reckless? To tell the others that the tide is going out and that the boat is riding lower. If she had shown up much later, the roughly six or seven guys just chillin' out would have been stranded at sea and, eventually, drowned.
In Guilded Age, the Peacekeepers decide to come to the aid of the World's Rebellion's champions when the beast the cultists let into the world attacks them.