In the comic book series ElfQuest, when two elf tribes — the Wolfriders and the newly-introduced Go-Backs — join forces to wage war against the trolls of King Guttlekraw, the elves form a grudging alliance with the trolls formerly led by the late King Greymung, who have been enslaved by Guttlekraw, even though Greymung's trolls have been the hated enemies of the Wolfriders ever since betraying them back at the start of the storyline. The Go-Backs, meanwhile, would never have thought of teaming up with trolls of any kind, period... But it's probably only because of this uneasy alliance that Guttlekraw's trolls are defeated.
Tom Strong spent nearly a dozen issues befriending and helping various entities and past enemies. The scenario used them all.... to deal with a ludicrous, never before/ never again seen, dead-in-one-issue menace. This may or may not have been a parody, though.
The climax of the Lucifer comic involves the invasion of Heaven by a vast army, Lucifer himself forced to gather together a truly motley group of allies from past stories to defend it. Unfortunately at least one of them wouldn't mind killing him, and none of them particularly like him (or heaven for that matter).
Grant Morrison's run on JLA ended with the entirety of Earth gaining superpowers and joining with the Justice League to fight a potentially galaxy-destroying menace.
This was also something of a bookend moment, as Morrison's run began with an arc where all Earth's people hold up lighters, matches, and torches in order to defeat a Martian invasion (Martians in the DCU being weak against fire.)
Nearly everyone the Jaime ReyesBlue Beetle had befriended (and a few of the previous Beetle's old friends, but oddly not the team he was affiliated with at the time) came to the aid of either Beetle himself or his family and closest friends as he fought off The Reach during the "Endgame" arc.
The first story arc of Kurt Busiek's run on The Avengers sees virtually all of the (still-living) former Avengers teaming up to fight Morgan le Fay.
In the What If?: House of M one-shot, instead of saying 'no more mutants', Scarlet Witch utters 'no more powers', and the result is that every super-powered being throughout the Marvel universe loses their powers. The Red Skull uses this as an opportunity to take control by using the Cosmic Cube, assembling the Hand and HYDRA to use as his army, murdering the remaining active Young Avengers and the Fantastic Four, and announcing his intentions to bring about the Fourth Reich. Iron Man creates new armors for the remaining Avengers to wear in order to challenge him, and Cyclops rallies the X-Men, armed with Shiar tech, to take out the Red Skull. This fails, resulting in heavy casualties for both the Avengers and X-Men. Peter Parker enters the fray, announcing to the citizens of New York, who were cowering in the shadows, that they can no longer stand by and wait for heroes to save them, and that they are all needed in order to save the world, because it is the responsibility of everyone to stop injustice, powers or no. Peter then leads an army of New Yorkers, many of whom stand no chance of winning (children, the elderly etc), into battle with the Red Skull and his army. The heroes win.
In Red Robin Tim takes advantage of Ra's al Ghul expecting him to take a leaf out of Bruce's book and try to defend Gotham against the League of Shadows without reaching outside the Batfamily for help. He calls in Superboy (Kon-El), Kid Flash (Bart), Wonder Girl (Cassie), Manhunter (Kate), Huntress, Manbat and others and thoroughly dismantles the plan Ra's had set into motion.
In Batgirl #23 Stephanie pulls this by calling in various super-powered girls to help her defeat the Reapers, even trusting Supergirl's superhearing enough to trust that she'd hear the word "Shazam!" from wherever she happened to be at the moment...which happened to be the Hall of Justice.
Minor example in Birds of Prey #111 which deals with Oracle going undercover at a tech convention and ending up in a tight spot with Calculator and Hellhound. Back at headquarters, Misfit gets the idea to text the entire convention list and send them all to Oracle's aid.
When The Avengers are facing their Darkest Hour during Avengers Disassembled, Hawkeye announces to Captain America that some heroes have arrived outside the Avengers Mansion. Cap steps out to see every former and reserve Avenger has shown up to help.
In Serenity: Leaves on the Wind, Mal tracks down the Operative to help him rescue Zoe after she's arrested by the Alliance. Nobody is happy about this.
The 20th and final issue of the IDW PublishingSamurai Jack comic book ends with an older Jack leading a resistance movement against Aku's fortress. Among the group are many of the allies Jack made and the people he helped out throughout the run of the original show, including the Woolies from "Jack, the Woolies, and the Critchellites", the astronauts from "Jack in Space", Extor from "Jack and the Ultra-robots", and even the baby from "Jack and the Baby" as an adult.
The final issue of Godzilla: Rulers of Earth has Godzilla surrounded and hopelessly outnumbered by an army of powerful alien kaiju, he's exhausted by weeks of continuous fighting, and the mechs that were assisting him, Moguera and Kiryu, have just been destroyed. But being the Determinator that he is, he lets out a truly awesome Mighty Roar as a defiant battle cry. Cue thirteen other kaiju charging in to help him out in the Final Battle for Earth.