Angel's finale, with Angel asking the hated and usually villainous Lindsey to help his team against the Big Bad, although he then, through Lorne, betrayed Lindsey. Illyria was also a semi-unlikely ally.
Farscape: past one-shot enemies were called upon to help the main characters rob a bank.
And at the end the Big Bad from the previous season shows up to save the day.
One season finale for Stargate SG-1 had the Tau'ri, Free Jaffa Nation, Asgard, and Lucian Alliance (the last only agreeing to help after Teal'c beats the crap out of the leader's guards and points a gun at his forehead) band together to stop the Ori from gaining a foothold in their galaxy. In a rather shocking case of The Worf Effect, the Ori outright slaughter the Milky Way alliance.
Made even worse by the fact that the Asgard were one of the most advanced races in the Milky Way, and even THEY were trying to desperately think of something fast to stop the Ori.
A more successful alliance recently happened in Stargate Atlantis, where the Atlanteans, their enemies the Wraith, and their allies the Travelers successfully destroy the Replicator homeworld, apparently losing only one ship in the process.
In Stargate Continuum, SG-1 (in F-15s) are attacked by gliders. They are saved by a squadron of Russian MiGs, possibly one of the few cases in Western fiction where incoming MiGs are a good thing.
The three-part ending of Ultraman Mebius both subverts it and then promptly plays it straight. When the alien Empera-seijin sends his army down to Earth to wear down the heroes, eventually rendering the hero near comatose and destroying most of their weapons and super-vehicles, the heroes get the aid of their various allies they've made over the season. Only to have it amount to nothing as Empera arrives and kicks the crap out of them with a few flicks of his wrist—eventually killing The Lancer and Ensemble Dark Horse before killing the hero in a single blow. A few of them get better, and get a true Gondor Calls for Aid from the Ultra-Brothers and a few of the slain are Not Quite Dead, and the world is saved.
Ultraman Gaia also gets one, though its far more effective. All of Earth's monster awaken to fight Zogu's (aka The Root Of Destruction) army after Gaia and Agul are defeated, the military ultimately helping them. In the end, they lend some of their energy to Gaia and Agul to revive and super charge them, allowing them to put an end to Zogu once and for all.
My Name Is Earl, "Camdenites": Earl rounds up just about everyone he's ever helped with his list to help him right Joy and Darnell's trailer.
In the finale of the fourth series of the new Doctor Who, Harriet Jones, the morally ambiguous former Prime Minister (Yes, we know who she is.), finds she needs to call on the Doctor despite the fact he's the one who deposed her. She even sacrifices her own life to serve as a distraction for the Daleks so the call for aid can get through. However unlike other examples it is more obvious the Doctor's companions will help.
In the 2011 mid-series finale, the Doctor does this again when he needs to rescue Amy and her daughter, by gathering people who owe him debts. Some of them we've seen before, such as the pirates from "Curse of the Black Spot", the space-spitfires from "Victory of the Daleks" and Dorium the black marketeer from "The Pandorica Opens". Others are introduced in the same episode.
In "The Wedding of River Song" there's an especially touching one when River explains they've constructed a distress beacon to broadcast "The Doctor is dying. Please, please help." and the Doctor comments "That would mean nothing to anyone." To which River replies
River The sky is full of a million million voices, saying, "Yes of course. We'll help." You've touched so many lives, saved so many people. Did you think when your time came you'd really have to do more than just ask?
Part One of the Series Finale of Battlestar Galactica ends with Adama putting out one of these for the mission to rescue Hera from Cavil's Cylons. Every character we know from the fleet is called upon to help with the mission if they so choose, including those Marines that were imprisoned after supporting the mutiny against Adama earlier in the season.
Babylon 5: The cast spent a good amount of time gathering all of the younger races, and several of the older ones, for their battle with the Shadows and Vorlons.
From the backstory, there's the Dilgar War. At the start of the war the Abbai called for help from anyone willing to fight, but the few who answered the call (League races that hadn't been attacked yet and a few Narn related to the ones slaughtered by Jha'dur) didn't have the strength to stop them. Then the Earth Alliance answer the call, stopping the Dilgar out cold with a growing strength that already matched theirs.
Again from the backstory, the Earth-Minbari War. According to the Expanded Universe Earth Alliance decided it was time to cash the life debts owed by the League, and called for their help. It was subverted; most of the League races sent condolences, the Brakiri were organizing a fleet but changed their mind when a Minbari warcruiser jumped in bombing range of their homeworld and powered up the weapons before jumping out, and the Drazi switched their support from combatants to heavy weapon technology plans after the first wave of their reinforcement fleets disappeared in hyperspace. This perceived betrayal is half of the reason of the human attitude in the series (the other being the fact nobody knows why the Minbari surrendered as they prepared to destroy Earth).
Subverted in Robin Hood: In the finale Robin and the gang are under siege at Nottingham Castle and they manage to sneak one of their people out in order to fetch King Richard's army who are rumoured to have landed back in England. However, the Sheriff soon informs them that Richard is in fact held hostage in Austria and no help is coming.
A strange corporate example occurs at the end of Season 3 of Mad Men. When Sterling Cooper's British overlords at Putnam Powell & Lowe decide to sell the firm to McCann Erickson, Roger Sterling, Bert Cooper, and Don Draper instantly realize that they (and Don's creative talent) would be sidelined at McCann's "sausage factory" of an ad agency. As a result, they rally a number of people to start a new firm: they get Lane Pryce — who had had something of a strained relationship with them as PPL's representative in New York — to fire them all in exchange for making him a partner in the new firm; they pull in Pete Campbell, whom Don still regards as too ambitious for his own good, to get his account skills and contacts at the lucrative Richardson-Vicks account; they get Peggy Olson, whom Don had recently insulted, to join as head of Creative; they get Harry Crane, the only man in the office smart enough to see the potential for having a TV division, as head of Media; and finally, they bring in the formerly-exiled Joan Harris as office manager, who scrounges up materials and people for the agency. And then, in a Midtown hotel room:
An inversion for the series finale of Seinfeld. When the four main characters are put on trial for violating the Good Samaritan Law, a long list of significant characters from the show's entire run are called as witnesses... for the prosecution.
Series Three of Blake's 7 ends with the crew of the Liberator calling on the Federation to defend against the Andromedan spacefleet — and defending the pass on their own until help can arrive. The relationship between the allies was... not nice, shall we say?
Many times in the finales of the Power Rangers series. But the most memorable is the one for In Space series. In it the rangers are beaten and Andromeda issue an ultimatum to surrender themselves or she'll kill the citizens. When zero hour comes, who should actually stand up to her but Bulk and Skull proclaiming themselves to be Power Rangers More citizens of Angel Grove start doing likewise, inspired by their bravery. Eventually the Power Rangers themselves join in and a full on battle takes place between the citizens and Andromeda's forces. What makes it memorable is that its the non-powered characters that ultimately help achieve victory.
The movie Let's Go Kamen Rider. Faced with the Great Colossus that was stated to be unstoppable until it's destroyed everything, ALL of the Riders (whether a hero, villain, or neutral) from the series' 40-year history appear, in order to help finish off the Colossus with the All Rider Break. If anyone's counting, that would be 90 Kamen Riders, more or less.
Once Morgana takes over Camelot at the end of season three of Merlin, Merlin writes to Lancelot, asking for his assistence. Lancelot arrives just in time, with Percival in tow, to save the company from a band of immortal soldiers by causing a rockslide.
Done in the episode The Last Dam Jobof Leverage. Notable in that it is one of the few examples where the heroes are clearly not okay working with their enemies.
Invoked and subverted in Star Trek: Deep Space Nine when a combined Federation, Klingon and Cardassian fleet about to face an overwhelming force of Dominion warships emerging from the Bajoran wormhole are unexpectedly joined by Romulan warbirds decloaking and announcing they are there to assist. The subversion comes when the Cardassians abruptly peel off and join the Dominion fleet as it proceeds to Cardassia, with Gul Dukat announces Cardassia has allied with them.