Winx Club: In the fourth episode of season two, Amentia tries to force one of the major characters into marrying her. Brandon effectively lucks out of it, and in the 24th episode of the season, Brandon and Sky decide to get Amentia's help in allowing the Specialists' ship to get inside the Underrealm to rescue Bloom. Also, in season one, a few nymphs from one of the filler episodes warned Alfea of the arrival of the villains during the final battle.
The fairies and witches joining forces against the Trix.
Justice League Unlimited's "The Return" had the entire League trying to help out citizen Lex Luthor (he got a pardon in a previous episode of Justice League) who was being targeted by AMAZO, an unstoppable robot. They didn't like it but they had to try and stop AMAZO especially after it looks like he disintegrated Oa. He didn't he just warped it out of his way. Once the misunderstanding is cleared up, he warps it back.
The final two episodes of X-Men: Evolution were the main cast calling on aid from almost every mutant from the series to stop Apocalypse.
The fifth season finale of the 2003 Teenage Mutant Ninja Turtles cartoon involved the turtles calling on everyone they'd ever worked with, and a few they'd fought against, to help stop the threat.
In Avatar: The Last Airbender, during the Day of Black Sun, various allies of the Gaang from the Earth Nation show up to help out. It's also one of the few cases where this gambit fails almost completely. However, in the series finale, other characters such as Iroh, Jeong Jeong, Bumi, and Pakku, to name a few, show up to help fight the Fire Nation.
The sequel series, The Legend of Korra, has General Iroh of the Fire Nation (Zuko's grandson, not his uncle) responding to Republic City's call for help in the battle against the equalists. Iroh brings the whole United Forces armada, in two waves (the second led by Bumi, Aang and Katara's son).
Season 4 of The Batman involves an Alien Invasion with Gotham City at ground zero. There's a breakout at Arkham Asylum, and the Gotham City Police Department arrives to find the inmates fighting the aliens, the Joker being especially angry that the aliens are invading his turf. When his lieutenant asks Gordon what to do, he orders his men to back the supervillains up.
The penultimate episode of Teen Titans, "Titans Together", had every young hero the team had encountered over the past five seasons coming together to help battle the Brotherhood of Evil.
In the last story arc of the first season of Filmation's Flash Gordon, Flash, Barin, Thun, and Vultan put out a call to the various allies they've made for a final united battle against Ming. It's pretty awesome.
ReBoot has Bob call for help from Megabyte and his viral army when the CPUs are outmatched by the web creatures. It turns out Megabyte was deliberately waiting in order to let the web creatures cripple the CPU forces so that Megabyte would have an easier time taking over after his inevitable betrayal.
This was also somehow used in the episode "Bookstore Bogus", where Bogus calls on the help of Little Red Riding Hood, the Big Bad Wolf, and Humpty Dumpty to help him save Rapunzel from the clutches of his sworn enemies, Ratty and Mole.
Ben 10: Alien Force does this in the season 2 two-parter finale, where Ben's team goes and summons all of his allies that he had encountered over the series (and Darkstar) to help fight against the Highbreed Alien Invasion.
The Avengers: Earth's Mightiest Heroes! does this twice. In one episode Annihilus attacks Prison 42 with his army of bug creatures and traps the Avengers and S.H.I.E.L.D. agents stationed there with no way to escape or call for help from the outside. So they end up releasing most of the super villains being held there to help fight off the invaders. Most, except Baron Zemo, who Captain America knows he can't trust. Also in the series finale when Galactus and his heralds come to devour the Earth, the Avengers call on almost every superhero that has appeared on the show so far.
In the season finale of ThunderCats (2011), Wilykit and Wilykat, realizing that the others are going to need help during the battle against Mumm-Ra's forces, go off to recruit nearly every friendly person or group the Thundercats had met during their journey thus far. They come back to provide The Cavalry at the crucial moment.
This is comically Double Subverted when they first come in: the big fanfare is for them to bring in one guy to help, but then we find out there were more in their Bag of Holding, he just didn't want to go in it.
The second season finale of Young Justice has every single live superhero showing up to help save the Earth.
Happens on South Park when the people from Jersey Shore stage a take-over of state-wide magnitude. After their pleas to the government fall on deaf ears, the inhabitants turn to Osama bin Laden of all people. And they get his support.
In the season finale of Gravity Falls, Dipper and Mabel resort to asking for help from an unlikely source, Jeff and his gnome army, who in the first episode acted as the twins' enemies, in order to defeat Gideon. They lose, resulting in a few honestly very sad scenes before things are turned around.