Ranma ½: Multiple times, both ways, between Ranma and Akane, Ranma and Ukyo, and Ranma and Shampoo. Seen mainly in the Ranma-Akane interaction, since they only show how much they care for each other when one of them is endangered, and they always have an excuse to cover up their very apparent jealousy.
In Samurai Pizza Cats, Polly constantly denied her feelings for Speedy, insisting they were "Just Friends" — even though she constantly got jealous of the time he spent with Lucielle. She even said she didn't like him, he was too goofy in the final episode. It took him almost dying trying to destroy a comet that she finally realized her denial and that she, in fact, loved him. They get together in the end.
In the anime version of Angelic Layer, Tamayo treats Kotaro only as a close friend until he develops obvious feelings for protagonist Misaki.
In the Lovely Complex manga and anime, Otani has never been able to see Risa as anything other than a friend...until she's suddenly swooning over Maity-sensei.
Otani: You're always saying "Maity this, Maity that"!! That's why I'm so annoyed!! Riza: ...Why should you be annoyed? Otani: How should I know? Annoying things will, of course, annoy me!! Don't ever mention his name again!!
Tomoe from Kamisama Kiss is a regular ass to the heroine Nanami 95% of the time but should another man so much as even talk to her watch out.
FAKE: While JJ's affection for Dee does stir pangs of jealousy for Ryo, it's not until Diana kisses Dee that this jealousy reaches it's peak, causing Ryo to realize that despite his denial over his attraction to Dee, he can't stand the thought of anyone else kissing him. This conversation with Dee almost reaches a Relationship Upgrade until they're interrupted. Again.
In A Cruel God Reigns, Jeremy begins to realize he loves Ian just as Nadia is fiercely trying to get Ian back.
Elizabeth of For Better or for Worse had this problem: one plot arc involved her breaking up by proxy with Anthony before the school dance, only to get insanely jealous when her friend felt sorry for him and opted to take him instead. Infamously, this plotline was later echoed and writ large when Anthony got married and Liz lamented that she should have been the one he chose instead.
The Ponies Of Olympus series has this eventually happen to Rarity in regards to her feelings for Spike. Her obvious anger at Ran Biao for seducing him, plus her previous jealousy of his friendship Wavedancer, are both used as evidence by Sweetie Belle to get her to admit to herself that she likes him back.
Emma: The titular heroine's best friend Harriet confesses she is in love with Mr. Knightley. It instantly comes over Emma like a thunderclap that she is in love with Mr. Knightley!
The Princess Bride: The attraction that sparks off the jealousy is one-sided, but Buttercup first realizes that she's in love with Westley when the Countess Rugen starts ogling him.
J.R. Ward's Black Dagger Brotherhood series uses this trope in Lover Awakened. Zsadist is forced to realize he's bonded with Bella when his brother-in-arms Vishous provokes him. Basically, Vishous acts like he's going to go after her, at which point Z tackles him in a jealous rage, and Vishous explains to Z that he's invoking this trope while being strangled.
In the third book in the The Hunger Games series, Mockingjay, Gale reveals that he didn't realize he loved Katniss until another guy flirted with her.
She may not have ultimately been in love with him, but Katniss experienced a version of this in Catching Fire when she imagined Gale being the one volunteering for the Games and beginning a televised relationship with his fellow tribute.
In The Dresden Files, Harry first realizes and admits his feelings for Murphy after she went on a romantic trip with Kincaid and Harry felt like he wanted to kill him.
In General Hospital, for geeky Spinelli and fashionista Maxie, it took the arrival of similarly geeky Winifred for Maxie to realize her feelings for Spinelli. And then she slept with Johnny. Hey, it's a Soap Opera.
Subverted in Better Off Ted. Linda tried to induce this by introducing Ted to her ex-boyfriend... but he and Ted hit it off and become friends. Then Ted pretends to hate him when he realizes Linda was only trying to make him jealous.
Castle: This trope is the bread and butter of the show.
In Fawlty Towers, Basil and Sybil appear to have nothing but contempt for each other; however, Sybil gets very upset, when she thinks that Basil is pursuing a guest in "The Psychiatrist", and Basil frantically tries to prove that he isn't.
Community: Although Brita constantly insists she has no feelings for Jeff, she ends coming to terms with her feelings when Slater attempts to renew her relationship with him.
This is also subverted when Annie attempts to get Rich to join their study group. Jeff is seemingly jealous because of his feelings for Annie, but it's later revealed that he was being apprehensive simply because he hates how perfect Rich is, and once to learn how to manipulate others with similar perfection.
iCarly: Sam, if her actions at the end of "iSpeed Date" were meant to indicate her feelings for Freddie. Carly might end up doing the same in a future episode.
Teased a few times with Josh and Donna on The West Wing, but hammered home on Donna's part when Josh starts dating Amy Gardner.
In the series Eva & Adam, about middle-schoolers in love in Stockholm, Eva only realises she likes the new boy Adam when her best friend seems to say she may be interested in Adam.
Friends: In an unique example Chandler and Monica are already sleeping together but Chandler is crushed when she dates another guy and realizes they're about more than sex. Played with in that he clearly felt more from the beginning but it took jealously to vocalize it. (And Monica never wanted to go out with anyone else, she was set up).
NCIS: In the oft teased Tony and Ziva relationship, the two will usually deny any romantic feelings for eachother but will frequently show jealousy when the other person is in a relationship, i.e. when Ziva was with Micheal, Ray etc. or when Tony became involved with E.J. and Jeanne.
In Suits, Rachel kisses Mike after finding out he has started a relationship with Jenny.
In Raising the Bar, Bobbi gets jealous when Jerry is attracted to a professor he has brought in as an expert witness. He calls her out, and she finally wisens and acts on her feelings for him after pushing him away a few times.
In Leverage, Parker is hinted to be interested in Hardison, but nothing serious had happened between the two of them beyond casual Ship Tease. That changes when Hardison is shown getting flirty with one of the clients, and Parker reacts negatively to say the least. She breaks a bottle with her bare hands.
Played straight and then averted in That '70s Show. Hyde realizes he has feelings for Jackie, who has been pursuing him and trying to make him jealous. They then go on a date and realize there's not actually any chemistry between them. (...Then later they hook up out of boredom and become a couple for a time, until Hyde runs off to Vegas and drunkenly marries a stripper.)
Jackie also has such a moment with Kelso, when she sees him with his new girlfriend and yells at her, ‘Get off my boyfriend!’ She and Hyde have an argument about this later on, in which she explains she probably does have some feelings left for him, as he was her first boyfriend, but she loves Hyde. They stay together.
Caroline has one in the 3rd season premiere of The Vampire Diaries after seeing Tyler with a date during Elena's birthday party. Being what she is, she mind controls the girl to go away and makes her feelings for Tyler pretty clear. Cue Relationship Upgrade.
Subverted on the 2 Broke Girls second-season episode "And the Pearl Necklace." At the end Sophie, who had early rejected Oleg's gift of a toothbrush for him, and denied to Caroline that she was "exclusive" with Oleg, suddenly excludes his date for the evening from going back to the kitchen to see him. When Caroline confronts her about it, she still denies she's "exclusive" with Oleg.
Where I come from, you never throw anything away.
In a Season One episode of How I Met Your Mother, Robin witnesses Ted having a romantic moment with Victoria. The jealousy Robin feels forces her to admit to herself that she has feelings for Ted.
Also happens in a Season Five episode when Barney and Robin seem to have a no-strings-attached Friends with Benefits relationship. But when Barney witnesses another man ask Robin out on a date, he becomes visibly distressed and later attempts to fight the man who asked Robin out.
Star Trek: Enterprise. T'Pol denies her attraction to Trip Tucker until she notices that MACO Corporal Amanda Cole has an interest in him. She sleeps with Trip, then quickly backs off from the relationship (it helps that Cole never appears in any future episode) saying it was just her experimenting. Likewise Trip only realises he is in love with T'Pol (as opposed to merely attracted) when she enters an Arranged Marriage.
Paramore's "Misery Business" is an extended musical Take That to the former girlfriend in this situation. Including calling her a whore. Given that, according to Hayley Williams, it's based on events from her own life, it's a possible twofer...
Avril Lavigne's "Sk8er Boi" is a song about a girl who is in denial about her love for a boy she knows until he's already been claimed by another. Unusually for arcs invoking the trope, she doesn't get him in the end. Here the rival is the song's point-of-view character, possibly the artist herself — more normally we follow the fate of the girl having the epiphany.
Toby Keith's "How Do You Like Me Now?" is about a guitarist/songwriter gloating through the radio at the girl who rejected him in high school and went on to marry some rich guy who never has time for his family.
"Judy's Turn to Cry" by Lesley Gore (the sequel to "It's My Party (And I'll Cry if I Want To)") has the singer get Johnny back this way.
"Taken" by One Direction
You only want me when I'm taken
Tchaikovsky's Eugene Onegin has a tragic example: Eugene hadn't returned Tatyana's affections until after she married Prince Gremin, after which he realized that he had loved her all along. They meet, and it becomes clear that she still loves him as well, but she decides to remain faithful to her husband.
In Twelfth Night, when Orsino learns that his object of affections, Olivia, has gotten married, Orsino loses it — and it reveals that he's fallen in love with Cesario, his page boy, whom Olivia thinks that she has married.
Valkyria Chronicles plays with this trope when dealing with Welkin and Alicia. Welkin's mannerisms and (for the lack of a better word) obsession with nature put Alicia off in the early going, but as the game progresses, she begins to appreciate how Welkin's knowledge gets them out of rough situations. The anime, however, jacks it up to eleven.
El Goonish Shive: Sarah isn't in denial, but Elliot is, and he's dating Nanase. When Sarah finds this out, she runs off crying. Who steps up to the plate and fixes things? Nanase, of course, by dumping Elliot so she can move on and he can be with Sarah. It helps that Nanase's a lesbian and realizes later that this is why she never felt sexually attracted to Elliot.
Skin Horse: Marcie has started 'dating' Chick Magnet/Wholesome Crossdresser Tip. Slightly less effeminate Chris (who is begrudgingly forced to admit that he understands why Marcie is attracted to Tipa little too well) won in the end when his jealousy forced him to make a move for Marcie... which may have been Tip's intention all along. For the record, Tip's reaction to being dumped was essentually "K. Can we still have sex?"
Ron's jealousy over Eric was crucial to helping him discover his own feelings for Kim.
The convenient Mankey/Monkey concerns Ron had about Kim's earlier boyfriend, Josh... although Ron helped her somewhat with that romantic pursuit despite his misgivings.
The same happened to Kim herself, when she jelled against Ron and Yori. And Ron's growing friendship with Felix as well, which was especially amusing since Kim apparently has a "Ron Night" each week that both she and her parents are aware of and yet didn't consider that maybe they had a little more going on than simple friendship.
Subverted in the "Burning Low" episode of Adventure Time when it is made to appear that Princess Bubblegum is jealous of Finn dating the Flame Princess, even including an overrought "I thought I was ready for this!" after she found out. In the end, she's actually conflicted because she knows that extreme romantic feelings would stoke Flame Princess's flames to the point of destroying the planet, and she's aware that she'd consequently have to hurt Finn by forcing him to stop dating Flame Princess.
In the Family Guy episode Stuck Together, Torn Apart, Peter and Lois are advised by a relationship counselor to go on a trial seperation and date other people after Lois becomes frustrated with Peter's jealousy over her having coffee with an old boyfriend. At first Lois is supportive of the idea of a trial seperation, but when she witnesses Peter on a date with Jennifer Love Hewitt it provokes her into a jealous rage and Lois immediately takes Peter back.