Countless competitors in Professional Wrestling (which is fitting, as it's the business that named the term/trope). Often, a heel turn will stem from a face feeling as though he doesn't have the respect of the fans or his fellow faces; this is sometimes set up by having the face lose for several weeks in a row, finally lose his temper and blame the fans or other faces. Usually, the new heel will prove his heelishness with a sneak attack on a former face ally, often aided by his new heel allies. Heel turns by wrestlers who constantly switch between heel and face (Lex Luger and Paul Wight are two examples) don't have all that much impact on the fans, but when a long-term face goes heel it's a big deal. The biggest example of this might be babyface Hulk Hogan's heel turn at WCW event Bash at the Beach in 1996 after over a decade of superheroics, which shocked fans and generated mainstream attention. A wrestler who turns often enough eventually settles into "tweener" mode, where they're a face or a heel as the storyline demands.
One of the biggest shockers in the 1980s was the turning of longtime face André the Giant, who turned on Hulk Hogan in dramatic fashion, aligning himself with Bobby "The Brain" Heenan, a villainous manager of many known heels in the WWF. Heenan had not only been trying to dethrone Hogan for the past three years up to that point, he'd also sent many of his proteges after Andre over the years. Nobody saw the turn coming, and Hogan seemed as much horrified as he was stunned by it. Andre took matters further, challenging Hogan for the WWF World Championship at Wrestlemania III, and then proceeding to rip off Hogan's shirt and more importantly, his cross. Hogan, who in storylines (and apparently in real life) is a devout Christian (which denomination is unknown), was so shocked by this he actually started crying from the entire event and was so enraged by the turn that he responded to Roddy Piper asking if he would take Andre's challenge with an absolutely massive yes (one of the rare cases of a Big "YES!" not indicating happiness). Andre stayed heel for nearly the remainder of his career, not changing alignments again until 1990, retired around late 1990-early 1991, and died only 2 years after that.
The Rock established his popularity with one of these. After being introduced as a Boring Invincible Hero, his unpopularity with fans was used the rationale for him turning heel. It worked, and he won over the crowds to his side.
Years later, after a long run as a face, he played with this again. In 2002, despite being a Face, he was still getting booed and receiving negative reactions from the fans in some of his matches (such as the WrestleMania X8 match against Hogan). During the Summerslam 2002 buildup, it was learned that The Rock was leaving the WWE for a while to go shoot a movie. So he returned to the WWE in 2003 as a Heel.
However, The Rock's natural charm couldn't be hidden by his heel-ness and he often had to try really, REALLY hard to keep fans booing him. However, the fans could tell underneath the obnoxious villain, there was still the entertaining face everybody loved. By the time his match with Goldberg happened at Backlash '03, it seemed he just stopped trying to get fans to hate him and just accepted his role as a fan favorite.
By the end of WCW's life, writers were doing this with everybody whether it made sense or not, possibly hoping they could reignite the excitement of the Hogan turn with someone else. It didn't work but that didn't stop WCW from doing it every week. Fans were more confused than anything as they couldn't tell from week to week who was a face and who was a heel. By the time they pulled one with their cash cow, Goldberg of all people, fans simply stopped watching out of frustration and WCW went belly up not long after that.
Another famous tag team split occurred in the late 80's in the Jim Crockett NWA, when Lex Luger and Barry Windham were the top faces, feuding with Ric Flair's devious Four Horsemen stable. In the weeks prior to a huge tag team title match, the Horsemen constantly told Windham that Luger (who had recently left the Horsemen in a Heel-Face Turn) was only out for himself and would abandon Windham when he needed him most, teasing that Luger would turn on him and join the Horsemen. In the match itself, a badly beaten Windham tried to tag his partner, but Luger had just been knocked off the ring apron by Horsemen associates, thereby "proving" that he wasn't there when he needed him, and prompting Windham to turn on Luger and join the Horsemen himself.
There seems to be a distressing tendency for a wrestler's Face-Heel Turn to coincide with his winning a championship title. Supposedly, this is to maintain the "underdog" status of the face wrestler, and let the fans root for him against the Jerk Ass with the title. Sometimes the turn happens in the title match itself, if the 'underdog' wins the title through a particularly cheap method (use of foreign object, another wrestler interfering with the match, and so on).
Chris Jericho is a shining example of this. Jericho has never...EVER, won a World Championship as a face. However, when he was a face he played the perfect underdog - being screwed over by anyone with a shiny waist.
One prominent example of this would be "Stone Cold" Steve Austin's turn on The Rock at WrestleMania X-Seven, where he teamed up with his hated nemesis Vince McMahon due to desperation to become WWE Champ. Somewhat narmed by the Texas crowd acting as if it was a Heel-Face Turn for Vince, as Stone Cold is a local hero there.
After winning his first WWE Championship in 1999, it took Triple H eight years to win the title as a face for the first time.
Triple H was the Designated Hero for the main event against Chris Jericho at Wrestlemania X8 in 2002 mostly because 1.) many fans believed he Took a Level in Badass by coming back from his infamous quad injury like a true Determinator and 2.) he was feuding with Stephanie McMahon while she was in full-on spoiled brat mode and supporting Jericho in their match. He won against Jericho and then immediately started his heel turn by picking a fight with perennial fan favorite Hogan. Once he attacked Shawn Michaels, it was complete.
CM Punk's heel turn in 2009 started when he cashed in his Money in the Bank contract on Jeff Hardy. The fans were very pissed off because Hardy just won what was only his second world title and it was taken away from him. Punk started to enforce his straight-edge lifestyle on both Hardy and all of the WWE fans. He finally completed his heel turn one day when he hit Hardy in the head with a microphone and beat him up.
Punk would yet again turn heel in Summer 2012 after nearly a year as an edgy Anti-Hero face. This time around, his heel turn was facilitated by feelings of being overshadowed by stars like John Cena and The Rock despite being the WWE Champion. Punk attacking The Rock at the end of the 1,000th episode of Monday Night Raw started this turn off slowly, and said turn was made complete by the time Paul Heyman officially affiliated himself with Punk in early September.
When Kane won the World Heavyweight Title in 2010, it started a feud between him and underdog face Rey Mysterio Jr. Eventually he confessed that he attacked The Undertaker that Memorial Day weekend, fully completing his turn.
Batista finally turned heel in the fall of 2009 after nearly five years of being a face. Funny thing is, WWE had attempted to turn him heel twice before, but without success. They had him betray the Undertaker in 2007, but the fans still cheered for him. Then they had him mete out some Disproportionate Retribution to Shawn Michaels in 2008, but that didn't go over, either. Finally, they had him beat the shit out of Rey Mysterio Jr.. Since Mysterio is so squeaky-clean good, the best way to turn another wrestler heel is to have him pick on the Ultimate Underdog (as was done with both Eddie and Chavo Guerrero Jr).
The reason it was even more effective is that Batista and Rey were Heterosexual Life-Partners...seriously, the Ho Yay was palpable. Both of them were good friends of the late Eddie Guerrero, they had each others' back for years, and both were among the top babyfaces in the company. With the heel turn, and still armed with his customary Genre Savvy, Batista was on the cusp of real magnificence. (Too bad it ended almost as soon as it had begun.)
Even then, Batista was still cheered at the start, thanks to the backlash stemming from Rey's suspension.
During the infamous Invasion angle, everyone representing WCW and ECW became heels regardless of prior status. In particular, Booker T went from being a highly competent champion face in WCW proper to a fairly inept heel during the Invasion who couldn't keep his title unless guys like Steve Austin and Shane McMahon got involved.
On the other side, all WWF superstars were supposed to be faces. They forgot why X-Pac has his own trope and he was booed in his match with Kidman. He quickly blamed the fans and so at Summerslam every match was WWF vs. Alliance except his.
Along with RVD, the only other Alliance member who WWE could not get the fans to oppose, for completely different reasons of course, was Stacy Keibler.
Eve Torres spent five years of her career as a face and her heel turn came completely out of left field. She was being stalked by Kane for weeks and when John Cena rescued her from an attack, she passionately kissed him. She was supposed to be having a thing with Cena's best friend (and super over) Zack Ryder. The storyline seemed set up to have Kane manipulating Eve but it's likely WWE took note of the insane amount of heat she was getting. Thus the next week she abruptly announced she'd been using Zack all along and ended up getting branded a "Hoeski"note Zack Ryder talks in "Bro-speak", and has a running big boot to the corner called the "Broski Boot." by Cena in the middle of the ring. The turn paid off as she became WWE Divas' Champion for the third time before the end of the year.
AJ Lee had a heel turn that took six months to properly complete. After she accidentally cost beau Daniel Bryan the title at Wrestle Mania, he told her he wished she had never been born. AJ actually snapped and became a loose cannon, attacking her former best friend Kaitlyn and eventually developing some disturbing Yandere tendencies towards Bryan, Kane and CM Punk. A heel turn was teased when Daniel Bryan eventually proposed to her but she left him at the altar in favour of becoming the Raw General Manager. She finally turned heel properly at the last PPV of 2012 as she cost John Cena a match against Dolph Ziggler.
Trish Stratus was WWE's biggest babyface Diva ever since Wrestle Mania X-Seven when she turned on Vince. However three years later (notably when the equally popular Lita had returned) she turned heel at Wrestle Mania XX when she slapped Chris Jerichonote Who had underwent a Heel-Face Turn when he fell in love with her and allied herself with his Arch-EnemyChristian. Several critics who thought the PPV was disappointing praised the turn as one of the few high points of the night.
Lita herself also had to turn heel due to real life issues. She had been in a relationship with Matt Hardy and it was revealed she'd had an affair with Edge. When that became public knowledge, crowds began to chant "slut" and "You screwed Matt!" at her. Thus they turned her heel for the first time ever in her wrestling career and paired her with Edge on screen. This actually resulted in her abrupt retirement from wrestling a year later as she kept getting excessive abuse from Matt's fans.