30 Rock uses this in a Show Within a Show that Jack's girlfriend's grandmother watches. The villain, Generalissimo, looks exactly like Jack, so Jack uses Executive Meddling to make the Generalissimo a good man who loves old Puerto Rican women, scratching lottery tickets, and going to McDonald's to order only coffee.
Agents of S.H.I.E.L.D. excellently subverts this. After being revealed as a HYDRA agent and killing several people, Ward starts off Season 2 claiming to be making amends and clearly thinking he'll be accepted. However, it's soon made clear the team are never going to forgive him for what he did and thus his turn never takes (in large part because Ward's actual motives don't really seem to be all that pure anyway).
Fred does a FaceHeel Turn when becoming Illyria, and then ends up doing a Heel Face Turn later that season, though she's deeply, deeply unhappy about it.
Darla and Lindsey both flirt with a Heel Face Turn at various points, and Connor does a full Heel Face Turn.
G'Kar started out the Babylon 5Myth Arc looking deceptively like the heavy, and evolved into the conscience of the entire Interstellar Alliance. This was intentional misdirection of the audience.
Or, as with Londo, he simply changed because the situation had done so and remained the same inside.
The Minbari warrior caste leader Neroon was a true Heel Face Turn.
Bester appears to make one in Seasons 3 and 4, providing significant aid to the rebels, but later on reverts to his true allegiance: himself (and, secondarily, Psi Corps over and above the Earth Alliance government) Not before programming Garibaldi to function as a double agent, however..
Battlestar Galactica: Athena and Caprica-Six. Later, the Leobens, Sixes, and Sharons as a whole. Also, Lt. Kelly during the mutiny.
The Boys (2019): Starlight ends up helping The Boys and Hughie get his payback from A-Train.
Happens to Spike slowly over the course of four seasons. He started as part of the Big Bad trio (and the originalBig Bad before Dru was healed and Angelus came to play) in Season 2, came back for a brief but still antagonistic cameo in Season 3, joined the main cast as a Nominal Hero in Seasons 4 and 5, then just completely joined the Scoobies Season 6 onward.
On top of that, we have Faith, who did a FaceHeel Turn first, then came back and Wesley, who appeared (to the team) to do a FaceHeel Turn before later coming back—both on the Buffy spinoff Angel, though the former appeared in the final season of Buffy too.
Timid wanna-be villain Andrew got one in Season 7, at first motivated primarily by his fear of Buffy and her gang, but later becoming Giles' protégé and a respected leader among the slayers.
El Chompiras and El Botija, a couple of thieves with a long criminal career and several convictions, at some point in the series promise to left the criminal life, and they succeed, working honestly in an hotel alongside Botijas wife Chimoltrufia and with certain support of local policeman Sargento Refugio and Licenciado Morales (the local D.A.), who in previous sketches where their antagonists. Several episodes after that deal with their struggle for rehabilitation in society and/or the bad times they spent in prison.
Theres a similar two-part episode with El Chapulín Colorado in one of the western settings, when famous gunslinger Matonsísimo Kid makes a HeelFace Turn, sadly due to his criminal past the people in the town still wants to hang him thus forcing Chapulin to help him escape prison. The bittersweet ending left ambiguous if Kid is gonna keep his turn or, on the contrary, the experience convince him otherwise.
The Christmas That Almost Wasn't: The Big Bad Prune, after spending the whole film being a Child Hater and Card-Carrying Villain, finally recovers his forgotten childhood when he receives a sailboat he had wished for as a child; his Start of Darkness was indeed when he never got it and decided his childhood just didn't happen. The moment the turn occurs is as we see Prune gripping his sailboat at the end; and it's hammered home when he gives it to a little boy at the VERY end!
In Deadwood, Al Swearengen starts out as the villain of the series, stepping on a prostitute's neck in the first episode and quickly getting into a rivalry with the honorable main character, Seth Bullock. However, the more Hearst influence insinuates itself into the town, the more heroic Al becomes, forming an alliance with other sympathetic citizens and working to thwart the new Big Bad. He's still a total bastard.
The Fifth Doctor's companion, Vislor Turlough, joins the TARDIS crew as The Mole, under orders to kill the Doctor by the Black Guardian, but has a change of heart, defies the Guardian, and remains a loyal companion until his departure.
"Evolution of the Daleks": Dalek Sec, the leader of the Cult of Skaro, has one after merging with a human, coming to reject the Daleks' absolute xenophobia. The other Daleks unsurprisingly turn on him for this, and he winds up exterminated.
"The Poison Sky": Luke Rattigan eventually does this after learning that the Sontarans were only using him all along and had no intention of fulfilling their deal to take him and his students to a new world (General Staal smugly claims that the planet never existed in the first place). In his case, it leads to Redemption Equals Death, as he decides to atone by taking the Doctor's place in a Heroic Sacrifice to destroy the Sontaran ship.
"Journey's End" reveals that Dalek Caan had one after his last appearance. He does a face turn after seeing "the whole of time and space" and bearing witness to the destruction and devastation that his species have caused throughout history. Unfortunately, since he had become insane and babbling, it was a little too late by the time the Daleks had discovered his actions.
In "The End of Time", the Master realizes that much of his insanity was caused by the High Council of the Time Lords as part of a Time War-related gambit, and ultimately after fighting the Doctor for years joins him in defeating Rassilon and his council.
Although, by the Master's next appearance, she's back to her usual tricks.
"Let's Kill Hitler": After the Doctor has been poisoned by Mels, who was raised to kill the Doctor, he whispers something in her ear intended for River Song. When Mels figures out that she is River Song, she uses all of her remaining regenerations to bring him back to life.
"Into the Dalek": Rusty has a FaceHeel Turn and a Heel Face Turn within twenty minutes. Sort of. He's still a Dalek, but he only exterminates other Daleks now. The whole episode was about finding out if the Daleks were capable of this.
Both Crais, and then Scorpius start off in pursuit of our heroes before they end up joining the crew in Farscape. Aeryn is the original one to pull that move. When you see that video of what she used to be like three cycles ago ... whoah.
After Jon wins the Battle of the Bastards, Robett Glover joins him.
Sandor deserts Joffrey and the Lannisters in "Blackwater".
Although Tyrion was never a true villain, he spent the majority of the show's early seasons actively furthering the goals of House Lannister (one of the show's most morally dubious factions.) After being cast out and betrayed the entirety of his family (sans Jamie), he becomes a key ally of the (relatively) more heroic Daenerys.
Formerly a mercenary and member of the Second Sons, Daario kills Mero and Prendahl and joins Dany.
Qhono was (and in many ways, still is) a brutal rampaging warrior who wanted to rape and pillage everything in his sight, but now he's steadfastly loyal to Daenerys, who keeps his impulses in check.
In Garrow's Law, right at the very end, Sir Arthur Hill turns the tables on Lord Melville by exposing corruption at Garrow's instigation - and in the last scene he is shown giving Garrow and Sarah his blessing as they embark on a new life together. Quite amazing, given the animosity he has exuded throughout the series.
No mention yet of Sue Sylvester? Not only did she rank New Directions top amongst the other teams in Sectionals, she was also entirely responsible for the club staying open another year, despite having spent the past nine months continually trying to close it down.
Well, Sue only allowed New Directions to live because she loved hating it so much.
It's implied that Eliot Spencer from Leverage had one of these at some point pre-series, although we never get much in the way of details.
Lost: As shown in flashback, Juliet had actually gotten sick of the Others some time before she was even introduced on the show. She took the first opportunity she could (being made The Mole) to betray Ben.
Ben himself finally has one part-way through Season 6, but only after having everything he cared about—his daughter, his authority, and Jacob—taken away from him.
The Freighties didn't really know that they were on the wrong side at first, but after witnessing some of Keamy's atrocities, they switch to the Losties' side pretty quick.
Likewise, Charles Widmore, the former Big Bad of the series, suddenly stops trying to take over the Island and returns to fight the evil of the Man in Black after an off-screen meeting with Jacob.
Sawyer too. In the beginning a true Jerkass he gradually becomes more and more likeable before completing the Heel Face Turn in Season 5.
The argument could be made that he actually made the Heel Face Turn in Season 1, when he told Jack about Jack's father.
Layne Price in Mad Men moves in this direction when he agrees to Sterling, Cooper, and Draper's conspiracy; he completes it with gusto when he delivers his "Happy Christmas" to St. John, who had been pushing him around.
Occasionally used in the 60s spy series The Man from U.N.C.L.E.. Sometimes the U.N.C.L.E. agents manage to subvert a THRUSH operative into working for them (e.g., the second season episode "The Arabian Affair", where Solo informs a THRUSH minion who is due to retire that THRUSH liquidates retired minions to insure their permanent silence), whereas other times the villain comes over to the good side for his own reasons (e.g., the second season episode "The King of Diamonds Affair", where diamond thief Rafael Delgado decides to betray his organized criminal allies, free Solo and the Girl of the Week, and help them escape).
Margaret Houlihan has a very well-orchestrated one on M*A*S*H, which takes place over the entire fifth season, with a pivotal episode called "The Nurses", which Loretta Swit says is her favorite episode. Two other important episodes are one where an old friend of hers visits, and tells her that not just her pierced ears have closed up, she has, after which she makes a deliberate effort to be more open with others, and the two-part episode where she is stranded with Hawkeye under fire. The great part of the story arc is that while the character softens, and becomes kinder, and warmer toward people, she does not lose her "regular army" bearing, and there's no retconning of her earlier coldness, so it is real character growth, and not a reboot.
The 1998 Merlin series has Lord Lot and Lord Ardent, who are originally generals opposing the main characters but later become their allies.
In No Good Nick, Nick starts out with the intention of robbing the Thompsons blind, but warms up to them and reconsiders her scheme over the course of the series, ultimately abandoning it at the end of Part 2.
Regina the Evil Queen attempts one as early as the second season, wanting to become a good mother for Henry. Although she gets hindered by her mother's manipulations, Regina is on the side of good by Season 3 — even getting to use Light Magic, which only good can use.
Jefferson was previously a lackey of Regina but gave it all up to raise his daughter (after it's implied his wife was killed during one of their jobs). Although he gets sucked back into his old ways by Regina, he's ultimately reunited with his daughter and gets to live happily with her.
Ingrid the Snow Queen gets a last-minute one - when she discovers that her sister regretted sealing her in the urn years ago and her dying words were to free her. Ingrid has unfortunately cast a spell that will destroy everyone else so Redemption Equals Death comes into play.
Ursula the sea witch is given back her singing voice by Hook and reconciles with her father, leaving Storybrooke to live happily with him.
Maleficent gets to be redeemed when she finds out she is going to be a mother. Although separated from her child, she abandons any villainy when they are reunited.
After being sexist jerkasses, the Lords of the United Clans from Season 5's Brave arc finally accept Merida fit to be Queen of Dunbroch after she saved Harris, Hubert, and Hamish from execution; in addition, they aid her into finding her father's Enchanted Helm and defeating King Arthur. Even the Dunbroch Witch accepts Merida as Queen; after threatening to cast a bear curse on her subjects, when Merida has finally the aforementioned helm from her late father, King Fergus, the Witch admits that Merida has proved her leadership in Dunbroch as a Secret Test of Character and gives her magical ale from the Seonaidh.
Zelena (the Wicked Witch of the West) finally undergoes some Character Development in the second half of Season 5 before redeeming in Sisters. She becomes more neutral again throughout the first half of Season 6, but by Where Bluebirds Fly she is definitely a hero.
The Red Queen turns out to be Will's former girlfriend Anastasia and, once realises what Jafar has planned for Wonderland, joins Alice and Will to try and stop him.
Amara was previously shown to be a Lady of Black Magic who made Jafar what he is. However once it's known she is Cyrus's mother and she's saved from her prison, she actively fights against him.
Our Miss Brooks: In "Mr. Whipple", Miss Brooks, Walter Denton and Mr. Conklin are so affected by the story of the eponymous Mr. Whipple, a man who hasn't has a bite of solid food for a week. It turns out Mr. Whipple is a bad tempered miser who's on a liquid diet. No matter, the show of kindness affects Mr. Whipple so much he undergoes a heel-face turn and donates the money for Madison's new gymnasium.
Perfect Assassins: Leo goes from The Mole who sells the heroes out to Greely to helping them after Greely orders them killed, thus going against their deal.
In Space actually had the Big Bad, Astronema, briefly join the rangers after finding out that The Hero Andros was her long-lost brother. While she originally intended to betray the Space Rangers and thus secure her place as Dark Specter's chosen heir, she ended up siding with them, and was even declared an honorary Power Ranger herself. While Dark Specter and Darkonda recaptured and brainwashed her into an even more evil Astronema v2.0, she was restored to her true self, Karone, in the Grand Finale and later took over the position of Lost Galaxy's Pink Ranger.
Another example from Lightspeed Rescue is Diabolico, who starts off as the Big Bad, then becomes The Dragon when Queen Bansheera reveals herself, then dies at the Rangers' hands and has his powers transferred to Impus, Bansheera's son. The latter becomes Olympius, and has no intention of sharing his power, which results in the other two Elite Mooks, Vypra and Loki, reviving Diabolico. In the end, Bansheera shows that she's willing to sacrifice all of them for her sake, and even forces Diabolico into killing Loki, his best friend who was blindly loyal to Bansheera. In spite of Bansheera's best attempts to make Diabolico Brainwashed and Crazy afterwards, his loyalties lie with the rangers from that point on until his second death. And the clincher? His ghost is what ultimately saves Red Ranger Carter from making a Heroic Sacrifice to defeat Bansheera forever, taking on that role himself.
Mystic Force has Leelee first, followed by 2 of the 10 terrors, and in the finale, Necrolai.
Also Jarrod and Camile from Jungle Fury, even though Jarrod was possessed he still had a change of heart.
Super Sentai too as some of the above's counterparts had their own Heel Face Turn. Burai, Tommy's Zyuranger counterpart and Rio and Mele, Jarrod and Camile's counterparts from Juken Sentai Gekiranger.
Go-Onger has the Big Bad Triumvirate themselves, of all people. It happens over time and even Reunion Shows. It starts near the very end of the regular series, when the surviving two tell the Rangers how to defeat their much more despicable boss after he betrays them. All three reppearing in the Sanzu River in Shinkenger after their deaths, they tell the Rangers where the movie villain is after having decided not to help the Evil Plan more because they couldn't be bothered than anything else. Fast-forward to Gokaiger and they're found in the prison for villains who have betrayed evil, containing every Heel Face Turning monster across sentai history, suggesting the three are considered actual 'good guys' now. Fast-foward more to 10 Years Grand Prix, where they genuinely help the Go-Ongers out of the goodness of their hearts for the first time and become true allies.
Also from Super Sentai: Ressha Sentai Tokkyuger has Akira Nijino, aka ToQ #6... aka Zaram, a Shadow Monster who defected to become the Rainbow Line's signal man. He is The Atoner but feels that Redemption Equals Death; he won't be fully forgiven until he dies for his crimes. Of course, we're not talking the most serious Sentai there has ever been; we find out that when he was Zaram his crimes fell straight into Poke the Poodle territory and that even a Death Seeker can be played entirely for comedy.
Torin, The Mentor, from Zyuden Sentai Kyoryuger is worth a mention. He was formerly a member of the Deboss Army, one of its highest-ranked members alongside his brother Hundred-Faced High Priest Chaos, destroying planets full of intelligent life. He made his turn after arriving on Mesozoic Earth, moved by the beauty of the planet and the potential of its inhabitants. Thus, he defected to recruit warriors to protect the planet from Deboss and its minions.
Selcan Hatun in Season 1. After spending much of the season constantly scheming against her fellow Kayis and collaborating with Kurdoglu occasionally, she ceases to commit any truly malicious acts after Kurdoglu conquers the Kayi clan from the inside, giving her a considerable change of heart. Season 2 goes out of its way to give this character her own story arc so as to make up for the atrocities that had been committed by her during Season 1.
The Tekfur of Karacahisar is set up to become a major threat to the Kayis after he embargoes the goods arriving at his city through the Hanli Market. But with a little convincing from Ertugrul and his alps, he lifts the embargo and commends them on saving his daughters life.
Show Me A Hero: Mary Dorman goes from being the housing project's most outspoken opponent to being one of its most substantial supporters after realizing that the people it helps are just regular people like her.
Lionel Luthor, in Smallville, who starts out as a ruthless businessman, bad father, and the Trope Codifier for Magnificent Bastard, undergoes an abrupt about-turn while in prison. His devotion to a life of poverty and charity work is short-lived, however, as a fight with Lex motivates him to return to ruthless tactics, ostensibly to actually protect Lex... And then he finds himself serving as a conduit for Clark's dead father, Jor-El, repenting for his past acts, devoting his life to protecting Clark, and suffering from really bad migraines. Unfortunately, all of this is so confusing that no one actually believes that he's a good guy when he begs them for help. He is eventually killed by his own son, Lex, after which Clark finally learns, once and for all, that Lionel had been on his side in the end, dying to protect Clark's identity as The Traveller from Lex. Clark acknowledges that Lionel was the third 'father' who had died for him, and honours him the same way he honoured Jonathan Kent - by pouring a handful of dirt on his coffin.
Metallo pulls one in Season 9, after a restructuring of his system purges him of the adrenaline rush that heightened his aggression and made him evil. Since the comic book character is still evil though, there's no telling if it will last.
More recently, Brainiac appears to have turned to the side of the angels. He now calls himself Brainiac 5 and is a member of the Legion of Super-Heroes, claiming that they and Clark "cured" him of his evil. He returned to the present to aid Clark by 1) preventing Smallville High's student councilor from trying to kill Clark at his high school reunion and 2) showing him his past, present and future, to help Clark get over his misplaced guilt over Pa Kent's death and his fear of the future, and move toward his destiny. Since this is due to reprogramming however, it may not count as genuine. Made even more muddled by the fact that this version of the character was reprogrammed into performing a FaceHeel Turn in the first place before the destruction of Krypton.
Tess Mercer is also attempting one of these as of Season 10. Given her HeelFace Revolving Door track record, it's hard to see where she'll end up. So far though, she's stayed loyal, even shooting down Earth-2 Lionel's offer to become the father she never had.
She stuck with it right to the end of the series, when in the Grand Finale, she uses her last breaths to give a last-minute verbal Take That! to Lex and wipe his mind with a neurotoxin, ensuring that he cannot use any of the knowledge he accumulated since the beginning of the series against the newly dubbed Superman.
Sylar on Heroes had the shortest Heel Face Turn in recent memory, lasting only partway through Season 3. And both a Heel-Face Mind Screw and a straight turn during Season 4.
More permanent examples would be Mr. Bennet towards the end of Season 1 and Daphne during Season 3, at least until she died.
Damar, Gul Dukat's Dragon on Star Trek: Deep Space Nine, stays on with the Dominion after Dukat goes off to do his own thing. Eventually, though, he realizes the Dominion is using Cardassia, and starts a resistance movement. He even winds up taking terrorist lessons from old enemy Kira Nerys.
This then leads the entire Cardassian fleet to turn on the Dominion during the final battle.
Seven Of Nine, a former Borg drone that the crew rescued from the collective, is the most famous example in the Trek franchise. Janeway is convinced that they can rehabilitate this drone back into the human being she once was, but it's not apparent early on, as she's initially adamant about returning to the Borg collective. Slowly but surely, Seven settles into an ordinary life aboard Voyager (well, as ordinary as it gets on that ship) and is given duties to perform as she readjusts to being a human being. Being a former Borg drone, she proves to be ridiculously capable and intelligent, as reliable as anyone on the ship. While this means Seven maintains a disagreeable, condescending attitude toward mostly everyone around her, and never exactly runs from her past (punctuated by the cortical implant on her forehead), she eventually begins to feel some level of remorse for her past actions as a Borg drone. After some time, she's embraced, in her own way, being human again, regarding Janeway and the Doctor as her mentors, and the Voyager crew as her "collective."
Species 8472, who debuted in the same two-parter as Seven did. They quickly make an impression as intimidating Starfish Aliens whose ships somehow outgun the Borg, which was unthinkable a few short years ago. Voyager somehow manages to survive their encounter with them, but Species 8472 return in Season 5 having set a trap for the Voyager crew while adopting human forms, and designing a section of a planet to look like Starfleet Academy. This time, rather than risk fighting them, Janeway and the crew manage to sit down and negotiate with them. Species 8472 actually come away from the meeting with a better perspective of Voyager, and a tentative agreement of peace.
Castiel had a very very gradual Heel Face Turn over the course of Season 4, although nobody knows until the final episode that the angels are the bad guys, so although the change in loyalty is gradual, the act of changing sides appears to happen quickly. It almost happens a couple of episodes earlier, but his superiors catch him, prompting a brief FaceHeel Revolving Door situation. In fact, we catch glimpses of that trope throughout the season, with Castiel frequently deliberating between his orders and his increasing sympathy for Dean.
Meg had one of these in Season 7, first working with the Winchesters simply because they have a mutual enemy in Crowley. Gradually, she develops an attraction to Castiel and sacrifices herself so the Winchesters and Castiel can escape.
In "God's Equation", Ray Morrison holds the other members of his Alien Abduction therapy group, Dr. Harriet Penzler and later Allie hostage, accidentally shooting Charlie in the process. After Allie helps him to realize that he is angry because he was sexually molested as a child, he lets the hostages go. He apologizes to Charlie for having shot him in "Dropping the Dishes" and joins the other group members in protecting Allie from the government in the final episode "Taken".
In the final episode "Taken", Dr. Wakeman tries to phone Tom in order to warn him that Mary is aware that he, Charlie, Lisa and Allie are hiding in Sally's old house in Lubbock, Texas. However, he is shot and killed by Mary before he has the chance to do so. Wakeman's attempt to help his erstwhile adversaries is motivated not by a realization that what he was doing was wrong but by a desire to allow the aliens' plan to reach fruition without interference.
Tin Man reveals that Azkadelia is not evil of her own free will but possessed by the original Wicked Witch. She ultimately casts the witch out and sisterly love saves her in the end.
Surprisingly, Ultraman Tiga is revealed to have experienced this in The MovieThe Final Odyssey. He was formerly one of the Dark Giants, fighting other Ultras and almost destroying the ancient civilization they protected. However, in his human form, he fell in love with a woman called Yuzare, who turned him to the side of light. Tiga defeated the rest of the Dark Giants afterwards, sealing them in stone.
Warehouse 13 has Claudia Donovan, initially seen as an antagonist. Early in Season 1, she hacked into the Warehouse to get to Artie, blaming him for her brother's victimization to an artifact and trying to have him help her bring him back. Once that succeeds, she's proven to be an overall good person, and becomes the Warehouse's Techno Wizard, and later a fully fledged agent.
Another example is Helena Wells, Big Bad of Season 2 who is forced to see the error of her ways and reforms, becoming an ally starting in Season 3.
Wonder Woman: In "Fausta the Nazi Wonder Woman", Fausta turns on her Nazi masters and helps Wonder Woman and Steve Trevor escape.