This is the state in which nobody seems to be allowed to reciprocate any romantic interest shown in them; since having some sort of love interest is standard for main characters in particular, this inevitably leads down the path of the Love Dodecahedron. Often, a story will begin with this trope, which then evolves into Cleaning Up Romantic Loose Ends and/or a less restrictive sort of Love Dodecahedron. Also, as the series ends, all bets are off and there might be some resolution here.
When fans support relationships that involve unrequited love, you're looking at Starboarding. This might be resolved —or, alternately, made even more awkward— if the unrequited makes their feelings known. If all the unrequited love is directed at the same character, they just might be a Mary Sue. The Ignored Enamored Underling may be working under someone Oblivious to Love. Any type 2 or type 5Love Triangle falls into this category, maybe because everyone's Oblivious to Love.
According to Takahashi, this is because all the side characters in the Love Chart were merely obstacles to Akane and Ranma's eventual hook up. Yes, even Mousse. The only reason Ryoga became the lone exception was because the fans demanded his happiness.
In the series itself, even Ranma and Akane's relationship could be seen this way; Akane doesn't know if Ranma is in love with her and Ranma's feelings about her remain a mystery, which is just as much a reason for their squabbling as the outside interference. Between Ranma "explaining" or otherwise denying any claims of attraction to Akane, and Akane's own combination of never even making a claim to deny and her Tsundere attitude covering up her more subtle signs of affection, it's no wonder that the manga canonically ends with them going right back to their usual behavior (claims that the manga's final line is a note that they do get married shortly afterwards is a falsehood, spread by the existence of a poorly translated online fan-dub).
Rumiko Takahashi also does this in Maison Ikkoku. Until the final season which finally untangles the complex relationships virtually all the love is unrequited. Even poor Kentaro (a minor character, the ten year old Only Sane Man) doesn't escape it. The first season ending song exemplifies the angst that can develop from this trope. Although in this series all of Takahashi's characters get a happy ending.
Peacemaker Kurogane: Saya has a thing for Tetsunosuke, Tatsunosuke is very protective of his little brother "Tetsu", Suzu was fiercely devoted to his late master Yoshida, but is now obsessed with Tetsunosuke, Tetsunosuke is the only person, other than his sister, that Susumu can relax around and show emotion towards, Okita is very fond of Tetsunosuke, Heisuke adores small cute things and frequently fawns over Tetsunosuke's cuteness, Yamanami takes a shine to Tetsunosuke, Hana tries to be a "mother figure" to Tetsunosuke...To sum everything up, the entire series is rife with Ho Yay and other implications, all of it focused on Tetsunosuke and all very much unrequited, though Tetsu tries to be a equal friend to everyone.
In Full Moon o Sagashite, Izumi loves Meroko who loves Takuto who loves Mitsuki who can't get over her dead childhood friend. Cue large helpings of Wangst (which isn't helped by Mitsuki's certain death in almost a year's time).
In Princess Tutu Drosselmeyer loves this trope. To explain: Ahiru has a crush on Mytho, who is loved by both Rue and Fakir but has no feelings of his own, and later in the series Fakir develops feelings for Ahiru. The couples sort themselves out in the end.
Asuka proclaims her love openly to Kaji, who in turn loves Misato, is not interested in an underage girl and sees Asuka like a daughter of sorts.
Gendo Ikari never moves on after the death of his wife, Yui, but this doesn't stop both Ritsuko and her mother from falling for him. He uses them professionally but never returns their affections.
Hikaru had a shy girl kind of crush on Toji and made him lunch every day, but he always remained oblivious. If he returned her feelings, he never showed it, either to her, or on-screen.
Fuyutsuki had feelings for Yui from the start, but soon enough she and Gendo were happily married.
It was implied in The End Of Evangelion that Maya held feelings for Ritsuko, but, like Toji, if she did return her feelings she never showed it.
Makoto has a big crush on Misato, which she obviously doesn't ever return, and in fact sort of uses him for favors and such.
And of course, let's not get into the fiasco that goes on between the pilots themselves. Seriously, let's not.
School Rumble. The entire series revolves around a number of Love Dodecahedra, all of which are never fully resolved by the time the series has reached its end.
Light in Death Note was using Misa and Takada's affections for his own purposes. And that's all the romance between any of the main characters that we see. Chief Yagami and Aizawa are married but their wives are almost never seen, and the flirting between Matsuda and Light's sister Sayu never goes anywhere (not that the Chief would have allowed it to).
Rem goes as far as to die out of unrequited love for Misa, despite the fact that Misa doesn't give a damn about her and it's suggested that she has thought about tricking Rem into dying for her when Rem tell her the story of Gelus, who did the same thing before Rem and for the same person.
Part II adds Karin being hot for Sasuke though as of later chapters she gave up on him, Ino blushing at Sai, and the revelation that Jiraiya liked Tsunade all along. Also we have Shiho who has a major crush on Shikamaru who is Not Dating Temari. And that's not even getting into Hinata's confession, which so far Naruto has not responded to and whatever the hell is supposed to be going on between Naruto, Sasuke and Sakura now (really, words fail). According to Word of God, Part II is supposed to have all the romance in it. In practice, this probably means from here on things are going to get even more complicated.
Then there's Kakashi's childhood, in which his childhood team had the same dynamic as his current one. Obito, Naruto's counterpart, was in love with Rin, Sakura's counterpart, who had feelings for Kakashi, who was Sasuke's counterpart. Then Obito "died", except that he didn't. However, Rin later did die, and her death is what caused Obito to become Tobi.
One Piecethrives on this trope. There will never be any actual romance in the series, every romantic interest in the series is unrequited and Played for Laughs. Camie at the very least has a serious crush on Hatchan that he either hasn't noticed or generally ignores, Hancock is head over heels for Luffy but Oda is not shy about emphasizing the feelings are unrequited, and Sanji seems to fall for every attractive woman he meets, and the ones he's been friends with for the longest haven't come any closer to even remotely taking his flirtations seriously. ( Though this is subverted later when the mermaids, especially one named Ishilly, seem to take a shining to him)
From Gundam 00... The fact that Tieria Erde came to have feelings for the first Lockon Stratos (Neil Dylandy) has been pretty much accepted by fandom; the exact nature and extent are heavily debated, though romantic love is a rather viable possibility. But while Lockon did care for Tieria, even if he'd survived the first season finale, his own issues would've made very hard for him to fully reciprocate his teammate's love for him.
Video Girl Ai is filled with this, just about every character has a case of this. It's done so well that it qualifies as a Tear Jerker. heck, even the pilot version (Video Girl Haruno) has quite a bit of this.
His next work, I's featured more instances of unrequited love.
The majority of Code Geass characters' loves are unrequited. Kallen, Shirley, Rolo, Milly, and Kaguya are all in love with Lelouch, each in their own ways. Lelouch had a crush on Euphemia, who was falling in love with his friend Suzaku, who was falling in love with her. However, Lelouch accidentally causes Euphie to die and Suzaku went bonkers for that. Lelouch also apparently loves his full-blooded sister a bit too much, but Nunally, in addition to loving her brother just as much, has a bit of a crush on Suzaku, who still mourns for Euphemia and has lots of Foe Romance Subtext going on with Lelouch. And Milly, even though she secretly loves Lelouch, is engaged to Lloyd who is pretty much Married to the Job ( she later manages to break the engagement off, though). There's also Nina's psychotic love for Euphemia or Rivalz's own crush on Milly.
Black Cat has a lot of this. Kyoko, Rinslet, Saya, and Creed all fell in love with Train, though Train is mentally / emotionally immature and apparently asexual, therefore rendering all of their feelings for him unrequited. And then there's Echidna who loves Creed, but Creed is too gay for Train to care. And then there's Janus' feelings for Rinslet, but Rinslet (at least in the manga) seems to be hesitant about returning his feelings due to her crush on Train. And then there's hints that Charden might like Kyoko, who explicitly states that she would like Charden if not for Train. And then there's Eve's crush on Sven, but Sven is much more concerned with Train (being Train's Heterosexual Life Partner and all)... it's all a mess.
It is widely believed that in the chapter "Snow Mountain Syndrome", where each member of the SOS Brigade was visited in their rooms by another Brigade member, the visitor was the one that the visited was attracted to the most. Keeping this in mind, it could then be said that Kyon likes Mikuru, Mikuru likes Haruhi, and Haruhi, Yuki, and Itsukiall like Kyon. No reciprocated feelings to be found.
This is further complicated by the fact that Itsuki uses who was seen in what room to form a diagram of the number to solve the formula and let them out of the mansion and then theorizes that it was Yuki who made the fakes to give them a hint. However this may not be true as the mansion they were in was set up to provide them with everything they wanted as soon as they thought about it, such as instantly finding stuff to make sandwiches in the fridge or an ice pack that wasn't there before...and right before fake!Mikuru appears in Kyon's room he was thinking about her coming in to see him. As it so happens, the diagram Itsuki drew also doesn't actually work. He drew an arabic "4" which couldn't be made given the positioning of the rooms, but a digital four (like an upside-down/flipped "h") still could have if different people had been in different rooms. Even ignoring the Snow Mountain Syndrome episode, it's heavily implied in the main series (and confirmed for the alternate universe) that Itsuki has feelings for Haruhi. Which means Itsuki likes Haruhi while Yuki and Haruhi both like Kyon.
England's got a 100-years-long and currently non-returned crush on America. The most obvious hint is the Valentines strip where England gives America some chocolates while blushing and being all embarrassed. America giving mixed signals (mocking England in one moment, claiming to be hurt because England didn't give him Valentine's chocolates in the next) doesn't exactly help matters, and neither does the fact that they're both Tsundere and * heavy* on the "tsun".
If you read a bit more on it, cases can be made for Prussia and Switzerland having unlucky Childhood Friend Romances towards Hungary and Austria, respectively.
It's also worth noting that according to the author's notes on the Greece/Japan/Turkey Love Triangle, both arrows pointing to Japan are one-sided. Even if you take the common view that Japan does reciprocate Greece's feelings or could potentially does so, that still leaves Turkey in the lurch.
In fact, it's pretty hard not to see Turkey as a third wheel in the strips he gets with Japan and Greece, considering how almost all the Ship Tease for the triangle goes to Japan and Greece. Add to that the beta version of the Drama CD revealing Turkey to have been in love with Ancient Greece aka Greece's Missing Mom who he eventually killed, and his potential Foe Romance Subtext with Greece who loathes him and is sweet on Japan in at least half of all fanworks and goes "I don't completely hate you..." at best in other fanworks, and well... let's face it, the poor guy is a Hopeless Suitor.
Bleach: the manga where Word of God has said he has no interest in writing a romance. As a result, any romance that does get mentioned or hinted at either ends tragically or isn't pursued (not even by the one feeling the emotions). Orihime is in love with Ichigo but has no interest in pursuing a romance with him. So, not only does he not know about her feelings, but she also has no interest in making him aware of her feelings nor does she expect to have a romance with him.
Heck, the whole Fighting Shounen Anime Genre can be very fond of this trope, as there are even known cases of characters whose only true purpose in the plot is to be a Hopeless Suitor. (Most of them are female, but males can qualify as well.
But in all fairness, Shounen Anime is not the only type of anime to do this, as Tenjho Tenge is also quite fond of this trope as well. The only known exception is Bob Makihara and his girlfriend Chiaki Kounoike are the only exceptions, with the former getting relatively little screentime and the latter being a fairly minor character to begin with.
Mahou Sensei Negima! can hardly avoid this when the object of affections is a ten or elevenish year old Adorably Precocious ChildOblivious to Love who thinks of them all as students/sisters/friends. The only other love relations seen so far are Tsukuyomi -> Setsuna, Asuna -> Takamichi, which he feels too inferior for, Evangeline -> Nagi and Natsumi -> Kotarou, who is also oblivious to love... mostly. Actually there are probably more examples, such as Fate's minions, but they still all fit.
And the Foe Romance Subtext between Fate/Negi. Negi is apparently the only person who interests Fate enough for him to smile slightly during their battles together. He also appears very entertained when Negi starts to prove himself to be a worthy opponent, his penchant for invading Negi's personal space, or how he apparently thinks about Negi when they're not fighting sometimes...
As of chapters 252 and 262, we actually get requited love with Konoka/Setsuna and Kotarou/Natsumi. Of course, the Unwanted Harem surrounding Negi is far from being resolved any time soon...
Katekyo Hitman Reborn!. Tsuna has a big crush on Kyoko, who only seems to see him as a really good friend. Kyoko herself is a whole lot more concerned with her Hot-Blooded brother Ryouhei (though she has also expressed slight interest in Gokudera). Gokudera, in turn, is obsessively devoted to (and very heavily implied to be crushing on) Tsuna, who sees him as a good friend (but is also scared of his aggressiveness). Yamamoto wants Gokudera to "open his heart to him" and obviously wishes to... further their relationship; Gokudera really dislikes Yamamoto, however, and hates how Tsuna seems more comfortable with him. Ryouhei mainly just cares about living life TO THE EXTREME (though he did have a small crush on adult I-Pin). I-Pin has a big crush on Hibari (who reminds her of her sensei). Hibari really only cares about having Foe Romance Subtext with Mukuro (and later, some with Tsuna). However, Mukuro is obsessed with having hisFoe Romance Subtext with Tsuna ("I want to possess your body" indeed). Ken and Chikusa are crazed fanboys over Mukuro, who tends to treat them like crap (save for one Pet the Dog instance). Haru loves Tsuna, who really doesn't see her that way, and constantly tries to discourage her. Hana has a crush on the future Lambo, but he barely knows her since 99% of the time he's stuck ten years in the future, leaving her to interact only with his infant self (to whom she's literally allergic). While Dr. Shamal may run after every girl who's ever passed within fifty feet of him, he has significantly more of a crush on Bianchi, who responds with a plate of Poison Cooking to the face. And then there's Bianchi herself, with an unrequited love for Reborn. Oh, and Levi's undying devotion to Xanxus is as unappreciated as Genkishi's is to Byakuran. Also Squalo has a very loud love for his boss, Xanxus. Gamma is also rather protective of Uni.
Depending on the canon, Sailor Moon and Tuxedo Mask fall into this for the first arc just with each other. In the manga, specifically, Usagi loves Tuxedo Mask and begins to love Mamoru. Mamoru loves Sailor Moon and begins to love Usagi, while also having memories of Princess Serenity. It takes the rest of the first arc for them to learn they are all the same people.
Rei loves Kaidou in the manga. She had a thing for Mamoru in the first season of the original anime and was deeply hurt when it was revealed he and Usagi were meant to be but she got over it.
Makoto loves her sempai and had a deep crush on Motoki in the first season of the original anime and the manga.
Minako loves Ace's alter ego in her manga and Alan in the original anime.
ChibiUsa has some... confused feelings about Mamoru.
Setsuna may have feelings for Endymion.
Seiya loves Usagi and possibly loves Kakyuu.
Queen Beryl wanted Endymion/Mamoru in the manga, which was apparently her reason for going to the darkside.
Original anime villains Ali and En love each other but also fall for Usagi and Mamoru, respectively.
Fiore from the Sailor Moon R Movie is the Ho Yay option and loves Mamoru, or is at least VERY protective of his "friend."
Prince Diamond is obsessed with Neo Queen Serenity and, by extension, Usagi.
Emerald is infatuated with Prince Diamond and insanely jealous of Usagi.
Cooan loves Rubius, who just uses her for his own gain.
Kaorinite wants Tomoe's pants.
Almost everyone in s-CRY-ed who is in love is unrequited or otherwise prevented - several make Heroic Sacrifices for a beloved they know is in love with someone else. One minor character has a Static Character girlfriend (who he loses and later wins back without her ever getting a single line), and Ryuhou, who does reciprocate Minori's feelings, won't act on them because of Scheris who is also in love with him. Straight Cougar practically quotes the trope name at one point in the Anime.
The manga Hatsukoi Limited is essentially a massive love 20-or-so-hedron. There are all these middle schoolers, and some high schoolers, and other random people... and almost all of them have an unrequited crush. Also, for much of the series, a character/unrequited love gets one chapter of screentime before being in the background for a long while; it feels really disjointed for the first few chapters. Despite this, a pretty good romantic manga.
In "Stop!! Hibari-kun!" Hibari is in love with Kohsaku, Kohsaku is in love with Rie, Rie is in love Shiina, Shiina is in love with Hibari. Needless to say, a lot of rejection occurs.
In Kämpfer, Shizuku, Akane and Mikoto all like Natsuru, who is hopelessly in love with Kaede, who in turn lusts for Female Natsuru. Probably the only reason why the girls haven't given up on him is that they know that hell will freeze before Kaede likes Male Natsuru. It's also not helped by the fact that there's certainly some amount of Mind Control going on and Kaede is revealed to be The Big Bad. It doesn't look good for anybody ...
In Digimon Adventure/Digimon Adventure 02 Tai/Taichi really likes his close childhood friend Sora, and for a while it seemed that she shared his feelings ... until she starts dating his best friend Matt/Yamato and marries him at the end of the series. Similarly, throughout the first half of Anime/Digimon Adventure 02 Gogglehead Davis/Daisuke crushes on Kari/Hikari but it is unrequited as she seemed to like her close friend T.K./Takeru—Davis' rival—moreso. Digimon Frontier also had it's share of unrequited love. J.P. really likes Zoe and while she seemed to value him as a friend she has a crush on Takuya instead.
Rampant in Saiunkoku Monogatari: Emperor Shi Ryuuki loves Kou Shuurei, who refuses to consider marriage because she's focused on her political career. Ran Shuuei loves Shusui, who loves Kou Shouka, who loves his dead wife Shokun. Emperor Shi Senka loved a woman he couldn't marry and had six concubines he couldn't love, leading to masses of resentment from all of them. There are a few instances of couples who manage to have requited relationships - To Eigetsu and Kourin, Sa Kokujun and Sa Shunki, Tei Yuushun and Sai Rin - but they're all secondary characters and they still have to work for it.
In Slayers, the shrine maiden Sylphiel has feelings for Gourry, who in turn is: a) oblivious to them, b) too stupid to notice anyway, and c) beginning to fall for Lina, the heroine, who he's been traveling with. There is also the sorceress Eris, who loves Rezo, the Disc One Final Boss of the first season/first several novels; he is far too obscessed with trying to cure his blindness to love another; another unrequited one for him is the anime-exclusive mechanical doll Ozzel. Meanwhile, Amelia has notable feelings for Zelgadis in the anime (not in the novels, given the No Hugging, No Kissing Rule by its author), but he either doesn't care or tries to avoid them. He does show care and concern for her at times and can talk to her casually, but he also gets fed up with her easily. Manga and game adaptations, however, have him somewhat reciporacate them. Also, in the novel, Luke loves his partner Milina, who doesn't give a flying crud. In the Hourglass of Falces manga, though, she sort of returns his feelings.
Full Metal Panic!: Kaname and Tessa both love Sousuke. Sousuke has huge mental problems (including Post-Traumatic Stress Disorder and IntimacyIssues) that prevent him from returning anyone's feelings until after three whole seasons, where he finally realizes he's developing feelings for Kaname. And then there's Nami, whom Sousuke later starts developing feelings for, except she dies, so so much for that. And Lemon had feelings for Nami, but she likes Sousuke. Then there's Tsubaki, who has a crush on Kaname, and feels a great rivalry towards Sousuke, who doesn't care. And Mizuki, in turn, has a (very) unrequited crush on Tsubaki. And then there's Leonard, who is in love/lust/whatever with Kaname, who dislikes him. To add to the mix, there's Ena Saeki, who had a crush on Sousuke, who thought she was a terrorist. More weirdly, Gauron, Sousuke's Ax-Crazy psychopath Arch-Enemy, is obsessively in love with him. In a creepy, violent way. And the two twins Gauron raised, Yu Fan and Yu Lan, desperately love Gauron (and each other). Then Gates has an unhealthy pedophile obsession with the twins. This whole thing gets a lot worse when you add how a huge portion of the students at school have secret crushes / are lusting after Sousuke into the mix.
Initial D coined the term "lonely driver" referring to guys who spend most of their time working on their cars and either never get a girlfriend or have a relationship go bad on them. Takumi seems to be exempt from this, but his friends have had it happen to them (at least Itsuki had a good relationship going for a while before Kazumi dumped him for an older guy she once loved). Keisuke falls under this as well because he turns down Kyoko due to his duties as an ace of Project D. Later on, Ryosuke was revealed to have had a relationship with a girl named Kaori who also liked another guy and was eventually driven to suicide
A cute slice-of-life gag manga called Wa! by Mahoraba's Kojima Akira eats, breathes, and sleeps this trope - the founding premise of it is that each character has a crush on another character, who in turn has a crush on another character, and so on until it makes a neat loop (with the addition of those two girls who eschew romantic hijinks for Seinfeldian Conversation). Notably, though, the Love Dodecahedron in this series is more comedic than dramatic, at least so far. Chapter 9 is a summary of all the unrequited feelings of the characters up to that point. Things started to change toward the end of the series, and by the end two of the characters have managed to hook up with the person they wanted to, and one pairing possibility is pointed out by a third character, leading to the two becoming obviously closer but stay in denial.
Tenshi Ni Narumon: Noelle loves Yusuke, who loves Natsumi, who loves her deceased brother Fuyuki-meaning Raphael, who loves Mikael, who seems to be interested in Noelle in 'that' way. And it's just the beggining. There's also Muse, who loves Eros, who loves Silky, who has weird fixation on Noelle]], etc.
In Karakuridouji Ultimo, there's Akitsu, who loves Rune, who loves Yamato, who loves Sayama, who later turns out to love Akira, one of the Evil Doji masters. Basically, you can't get anywhere with anyone in Ultimo, because they're either gay, straight or evil.
A staple of the Harem Genre since the main guy can only choose one girl, usually. Though almost all love is unrequited in Harem, relationships are much less complicated and messy than the Shoujo genre, which does have its own Harem. Given the fact that many Shoujo characters tend to be more fickle (inculding and sometimes escpacially the main heroine), love may acutally only be halfway-requited; the result though, is one hairy mess.
In order to maintain the No Hugging, No Kissing environment of Daily Lives of High School Boys, the creator actively sinks any possible hints of romance pairings — so Hidenori thinks Literature Girl is a weirdo, the Convex Mirror Girl is an illusion, Emi is Hidenori's counsin, and Karasawa is Habara-phobic. This does not prevent others to ship, though.
In Taiyou No Ie ,literally the only happy relationship was between Hiro's (dead) parents.
In Attack on Titan, Jean immediately falls head over heels for Mikasa whose eyes are only on Eren, who has no idea that Mikasa has feelings for him and may possibly have feelings for Annie though that changed once he found out who Annie really is. Word of God confirmed that Ymir has feelings for Krista but it's unknown whether Krista feels the same. Also Krista is crushed on by several male classmates, particularly Reiner but has yet to return any feelings for them.
In D.Gray-Man, Bak has a huge crush on Lenalee (to the point of being a Stalker with a Crush) but she doesn't know about his feelings. Rohfa falls immediately for Allen, who is a Celibate Hero. Likewise, Road made it clear she favours Allen by planting a kiss on his lips, who only saw it as a nuisance.
Ano Hana, to the point it's a Love Dodecahedron. Tsuruko has a crush on Yukiatsu but he is obsessed with Menma and considered Anaru to be Menma's Replacement Love Interest. Anaru likes Jintan but he is in love with Menma who returned his feelings but cannot be together due to Menma being deceased. In the end, Anaru/Jintan and Tsuruko/Yukiatsu are given a Maybe Ever After.
The ad campaign for the comic Young Heroes In Love explicitly plays with this trope, as does the comic itself.
The Super Young Team of The DCU has a love triangle (that's really more of a love line segment) along these lines; Big Atomic Lantern Boy loves Shiny Happy Aquazon who only has eyes for Well Spoken Sonic Lightning Flash who isn't interested in relationships.
Calvin & Hobbes: The Series: "Insanity is In The Air" runs on a Love Triangle of this nature - Andy is smitten with Maria, but she finds herself more interested in Calvin, who doesn't really care about love at all.
In Merlin fanfic A Very Hairy Situation, Merlin and Gwen are both in love with Arthur, who is in love with Gwaine. In the past, Gwen had a crush on Merlin, who didn't even notice. Due to his Magic Hair, Gwaine is actually a common target of unrequited love.
Film — Live Action
Easter Parade: At least until about a half hour before the end, Hannah loves Don, Don loves Nadine, Nadine loves Johnny (for his money), and Johnny loves Hannah. Then, of course, Don loves Hannah ("Why didn't you tell me I was in love with you?"), Johnny gives his blessing, and everyone forgets about Nadine.
In the Japanese DVD Sea Of Dreams, one short centers around a group of five friends spending a day together at Tokyo Disney Sea park. The story is narrated by a boy who has unrequited feelings for a girl dating another guy in the group. Throughout the short, it is revealed that all five teens have unrequited love for another in the group, but no one really acts on these feelings and life goes on.
Miao Miao. The main three characters play this straight, but even the side characters get this to a platonic degree.
In Red Riding Hood, Valerie's sister is in love with Henry, who crushes on Valerie, who only sees Peter. Also, Valerie's mother Suzette was in love with and had an affair with Henry's father and it's left ambiguous whether she loved Valerie's father at all.
In She's The Man, Viola likes Duke who has a crush on Olivia who likes Sebastian (who is actually Viola in disguise). Stalker with a Crush Malcolm also likes Olivia. Eunice likes Sebastian (who is actually Viola) with Toby habouring a crush on her. Thankfully, this gets all sorted out in the end.
In Sky High this is Mr. Boy, for Jetstream. Justified in-story, in that a hero like her could never notice someone in 'Hero Support' - not with the strapping, handsome, charismatic Commander around.
In Far from the Madding Crowd by Thomas Hardy, three men propose marriage to Bathsheba Everdene. Both of the first two men love her, but she refuses them both. She marries the third, then discovers that he's in love with another woman.
Steinbeck's entire East of Eden revolves around this trope, but with love in the general sense: father/son and brother/brother in addition to romance.
Bret Easton Ellis' The Rules of Attraction exaggerates this trope to the point of comedic ridiculousness, using a narrative which transfers constantly between ten or eleven promiscuous arts college students (though there are 3 central protagonists), all of whom have unrequited feelings for one another and/or more minor characters.
In Victor Hugo's The Hunchback of Notre Dame, deformed bellringer Quasimodo, increasingly unstable priest Claude Frollo, and lazy poet Pierre Gringoire are all in love with La Esmeralda (Gringoire even ends up married to her, though that doesn't mean he gets to touch her), who loves the soldier Phoebus, who's engaged to another woman but fine with boning Esmeralda on the side so long as it doesn't inconvenience him in any way and he doesn't have to acknowledge her in public.
Hugo likes this one. In 1400 pages, Les Misérables features one reciprocated romance with Cosette and Marius. The rest all end horribly. Fantine is ditched for a laugh by her feckless bourgeois lover Tholomyes - she becomes a prostitute to support their illegitimate daughter Cosette, gets sick and dies. Eponine develops a huge crush on Marius, but he doesn't even notice until she takes a bullet for him at the barricade and bleeds out in his arms. And in the (sort of) platonic sense, Grantaire doesn't care about the revolution but is deeply devoted to Enjolras, while Enjolras is too devoted to his glorious cause to put up with Grantaire's cynicism - until the National Guard comes to shoot Enjolras, and Grantaire is allowed to die at his side.
In the book Death Star, twenty years after the events of the Medstar Duology, the one character to feature in all three books is a surgeon who went from a teenaged prodigy to a cynical middle-aged man who's been permanently drafted. In Death Star he reflects that he still doesn't know if he loved Bariss Offee, a Jedi he'd known during the Clone Wars. Whether he did or didn't, her death hit him hard, and he thinks of her often. As the Medstar Duology featured Bariss as one of the viewpoint characters, the readers know that she was totally unaware - she'd known about another character who thought he loved her, and she'd spent a lot of time with the prodigy, but she never knew.
Great Expectations. The protagonist Pip has unrequited feelings on Estella, who was raised by her mistress to be an unfeelingIce Queen who breaks boys' hearts. Estella herself ends up in an abusive marriage. Pip, after realizing his feelings for his childhood friend Biddy, tries to start a relationship with her, only to learn that she was already in relationship with his Parental Substitute.
Tons of unrequited love exists in A Song of Ice and Fire, most of it spoiled by Arranged Marriages. The most notable case is probably Littlefinger's crush on Catelyn Tully, which lasted from their childhood into both characters' middle age, and may have been the root cause of The War of the Five Kings. In turn, Catelyn's sister Lysa is in love with Littlefinger, while Catelyn was in love with Brandon Stark (as was Barbrey Ryswell) but learned toSettle for Sibling Ned (who was in love with Ashara Dayne).
In her youth Cersei Lannister was infatuated with Prince Rhaegar but the king rejected Tywin Lannister's proposal they wed. The Prince was in love with Lyanna Stark, as was her fiancé Robert Baratheon — this was the cause of the war before the current one, Robert's Rebellion.
The Mad King, Aerys, would have rather wed Joanna Lannister than his sister, which combined with his increasing madness as he grew older made him come to hate his childhood friend Tywin Lannister out of jealousy.
I Capture the Castle: Stephen is in love with Cassandra, who's in love with Simon, who's in love with Rose, who's in love with Neil.
Ginny likes Harry, who has a crush on Cho Chang, who is dating Cedric Diggory. After Cedric dies, Harry and Cho try to date. It fails miserably. By this point in time, Ginny has moved onto Dean Thomas... Just in time for Harry to realize that he likes Ginny after all. Romilda Vane also tries to feed Harry a love potion.
Severus Snape has always loved Lily Evans. Lily eventually marries James Potter, who also has an unrequited crush on her for awhile.
Bellatrix quite clearly loves Voldemort, who isn't really capable of feeling or comprehending love.
Hermione and Ron love one another, but each has a few throw-away Love Interests; Hermione (along with the rest of the girls at school) has a crush on Gilderoy Lockhart, goes to the Yule Ball with Viktor Krum (who retains feelings for her after the fact, as well as expresses interest in Ginny at the wedding), and takes Cormac McLaggen as her date to the Slug Club party specifically to annoy Ron. Ron, on the other hand, has a thing for Fleur Delacour (who marries his older brother, Bill) and Madame Rosmerta, and dates the thoroughly obnoxious Lavender Brown to get back at Hermione for dating Krum. He also hilariously becomes infatuated with Romilda Vane after accidentally ingesting the love potions she put in Harry's chocolates.
Per Word of Gay, Dumbledore once had a crush on Grindelwald. It was unrequited, but that didn't keep the latter from exploiting it for their own gain.
Wicked: Melena cheats on Frex constantly, with her most persistent lover being Turtle Heart, whom Frex also seems to have a thing for. Whether or not Melena and Frex actually care about one another is debatable. Also, Boq has a long-time crush on Glinda, who seems to be in love with Elphaba, who may or may not return those feelings. Later on, Elphaba is in love with Fiyero, who is altogether confused about the concept of love (he was betrothed at the age of seven, so that's understandable), but seems pretty convinced that he feels the same way. His widow, Sarima, doesn't care about him too much, but seems to be furious when confronted with the fact that he was cheating on her (but she thinks it was with Glinda).
Twilight: Jessica likes Mike, Lauren likes Tyler but both boys (along with Eric and all the male population of Forks) are interested in Bella. Bella couldn't care less about them and falls for Edward, the only one who seemed to hate her at first.
And of course Jacob also counts, as he also falls in love with Bella, but she only considers him a friend.
Leah was engaged to Sam, until he imprinted on Emily. Losing her fiancé and best friend at once has naturally left Leah kind of bitter.
Song at Dawn: Arnaud is in love with Estela who is in love with Dragonetz who is in a relationship with Emerganda who is in love with him after he falls for Estela.
Live Action TV
Step by Step: Al Lambert was admittedly the hottest girl in Port Washington, Wis. (thanks to the stunningly beautiful Christine Lakin playing the role), and in the later episodes never was in want of a boyfriend. But during the first two seasons, Al was the tomboy (being only 12 at the time, and Lakin just entering her puberty period) ... and one of the earliest episodes — one where Urkel crash-landed in the Lamberts' yard — had her trying in vain to catch the eye of a good-looking sixth-grade classmate to take to the dance. Urkel helps her to realize that the boy didn't want her (because she was too tomboyish ... just wait five years and he would have been sor-ree!) and there were others more worthy of her time.
Then again, the same show also had successfull relationships: Sheridan and Delenn, Sinclair and Catherine Sakai, Garibaldi and Lise, and possibly G'Kar and Londo (that one's platonic). And its hinted Susan actually had feelings for Marcus but never admitted it (which only makes it more tragic).
In ER, Rocket Romano pines after Elizabeth Corday for a whopping seven years, from their first meeting until his death.
"The Ski Lodge" features the cruelest punchline of all these examples; after all the running around, Frasier is left to come to the painful realization that absolutely no one was interested in him.
On a more long-term basis, for the longest time Niles' crush on Daphne was firmly unrequited, long enough for him to finally give up on her and move on with someone else, just in time for Daphne to learn of his feelings and realize that she really was in love with him.
24's seventh season seems to have a pint-sized version of this going on: Janis likes Larry Moss, who likes Renee Walker, who likes Jack Bauer, who... probably likes Renee, or possibly only loves killing terrorists. Then season 8 shows that, yes, Jack does indeed like Renee.
On How I Met Your Mother Barney spent the entire fourth season secretly pining after Robin. Then, in the season finale, she says that she loves him back. Instantly, Barney's completely over his feelings for her and goes back to hitting on random bimbos. Then, when he finds out Robin lied about being in love with him, he goes right back to wanting her again. This all eventually leads to them doing a rapid-fire version of this trope.
Barney: Why are you so afraid of giving this a chance?
Robin: Because I'm scared of how much I like you!
Barney: Whoa, this is a bad idea.
Robin: Uch, you're right, it's a mistake.
Barney: Yes ... No!
Robin: I love you.
Barney: Let's be friends.
Robin: Okay, friends then.
Barney: I love you.
Robin: Let's get married!
Barney: No, you're smothering me!
The Russell T Davies-era Doctor Who. Mickey loves Rose, but Rose loves the Doctor, who Rose's mother Jackie flirts with on their first meeting, but he won't allow himself to fall in love, he is later loved by Martha (who the Doctor rejects at every chance), was loved by Sarah Jane Smith, and had been (at least one side of him) loved by the school Matron Joan. Oh, and Jack loves everyone. This is not at all the case in the classic series (even Sarah Jane's attraction for him is a Retcon), and becomes much less true in the Steven Moffat era. Although you have Rory's love for Amy (and Amy's ignorance of this) when they were young, and Amy's attraction to the Doctor (and his disinterest in her the the knowledge that she's engaged).
The tradition continues in Torchwood. Tosh is in love with Owen, who is either not interested or too proud to give her a chance. Gwen loves her boyfriend, but she loves Jack even more. Jack loves Gwen too, but is aware the she's taken. He is, in turn, casually involved with Ianto, who is pining for much more than that.
In Smallville, Clark with Lana in the first season. Chloe with Clark for the first half of the show, though she eventually got over it. Pete also had a secret crush on Chloe before he was Put on a Bus. Lex towards Chloe is hinted subtly (and less subtly in a deleted scene), Lionel is confirmed by Word of God to have a bit of this, and a Villainous Crush, towards Chloe. Lionel to Martha too.
In Glee Ken falls for Emma who pines for Will who's married to Terri. Until Will finds out Terri lied about being pregnant and then falls for Emma although Ken is still out in the cold.
Mercedes likes Kurt, who likes Finn, who is both interested in Rachel (who likes him back) and dating Quinn, who is cheating on him with Puck, who is dating Santana, who also has little side-flings with Brittany.
Kartofsky likes Kurt, who likes Blaine, who likes some random guy at the Gap.
Artie has a fling with Brittany because he still likes Tina, who has moved on to Mike Chang.
Artie misses Brittany the second time they break up. He tries (unsuccessfully) to win her back). At the same time, Santana is in love with Brittany, but is dating Kartofsky, who also seems to have a thing for Sam.
Will likes Emma but makes out with Shelby, has a sleepover (strangely, a literal one) with April Rhodes, and is in the process of divorcing Terri. Emma likes Will too, but is tired of trying to make their relationship work...so she moves onto Carl the dentist. Sue also develops a one-episode crush on Will.
Rachel likes Finn who is dating (and eventually breaks up with Quinn), so Rachel moves onto Jesse while still having feelings for Finn who ultimately decides he wants to be with Rachel. And this happens twice.
Quinn dates Sam, cheats on Sam with Finn, and then Santana breaks Sam and Quinn up so she can date Sam. After Sam and Santana break up, Rachel tries (unsuccessfully) to ask him out.
Jacob Ben Israel likes Rachel, which never changes. Also, Puck's crush on Lauren goes unrequited for quite some time.
In Primeval, the main character Nick begins to fall for Claudia who is then erased from the time line and replaced with an alternate version called Jenny who Nick also falls for until he's shot. Likewise, Nick's ex-wife Helen tries to seduce him but fails and then tries on his best friend Stephen who leaves her after he ''finally'' realises the sort of person she is- and then sacrifices himself. Abby falls for Stephen at the beginning of the show's run but is discouraged when she learns he already has a girlfriend (who is not Helen). Of all the relationships in the show, only the Connor/Abby arc comes to something - and even that took two whole series and the better part of a third.
The Vampire Diaries: Damon loves Elena, she doesn't love him back. He loves Katherine, who also doesn't love him back, but instead loves Stefan. Who is in love with Elena. In 2x22 and 3x09 Katherine reveals she loved Damon too. And both times, she says it after saving Damon's life. Subverted in Season Four when Elena transitions and admits to Stefan that her feelings for Damon have been magnified. This causes a mutual break up.
Community: The first few episodes seemed to offer a fairly predictable version of this, with Official Couple Jeff/Britta and Beta Couple Annie/Troy. But it eventually subverted this, in that after a while the characters decided to move on to other people instead of fawning over someone seemingly uninterested. There is still occasional Ship Tease between these pairings, but it's not a primary focus (and there's just as, if not more, Ship Tease between other pairings, such as Jeff/Annie).
As of the season 4 finale of iCarly, Sam has fallen for Freddie who has fallen for Carly who could be harbouring her own feelings towards Freddie due to a breakup after he saved her life in an earlier episode eventually lead to them breaking up because of Freddie was unsure if Carly really loved him and not just because he saved her life. In the season 5 premiere, the sequel to the season 4 finale has Freddie kissing Sam and returning the feelings Sam has for him.
C-drama Holy Pearl (loosely based on InuYasha) has this in spades. To begin: Mo Yin loves Rong Di who loves Xian Yue who loves Wen Tian. Wen Tian manages to buck the trend with Xian Yue's reincarnation Ding Yao, who loves him back. Until the very last episode, at which point it is played depressingly straight.
Bunheads. Truly loved Hubbell who loved Michelle until he dies whuile Michelle doesn't love anyone really, not even herself. Meanwhile Boo loves Charlie who's barely aware of her existance and all the girls except Boo are in lust with Godot who, again, is barely aware of them except for Boo with whom he's friends.
On The Big Bang Theory, Leonard started pining for his neighbor Penny the minute they meet. Penny thought of him as a good friend, and continued to date the Dumb Ass Jerk Jock type. Eventually, after a few false starts, they get together in season three...and then they break up because of The "I Love You" Stigma, specifically Penny not being ready to say the words yet. Then in season four Leonard gets a Romantic False Lead...er starts dating Pryia...and Penny begins to confront her feelings for Leonard, realizing she still cares about him and that he's a great guy, probably the most genuine man she's dated (and undoubtedly the smartest). And also still dating Pryia.
In Dracula, Lucy is in love with Mina, who is engaged to Jonathan, whilst Lady Jayne fancies Grayson who is pining for Mina, the look-alike of his deceased wife.
There's a strong argument to be made that the entire sub-genre of Pop Punk is built on this trope.
The They Might Be Giants song "Withered Hope" is about about a Love Pentagon that's like this. A Very Sad Sack loves Withered Hope, who has a soul mate, who loves a picture of a crook, who loves a motorcycle, who loves Very Sad Sack...
The Burning Hell's song "Grave Situation, Part 3" describes a love quadrangle involving "a man who loved a man who loved a woman who loved another man"
In "Living Next Door to Alice", most successfully covered by Smokie, the narrator has been in love with his neighbor for 24 years, but couldn't tell her. And now she's moving away. Meanwhile, the narrator's friend, Sally has been in love with him for 24 years, but he ignored her because of Alice.
Peanuts has this trope as the norm: Unrequited love is practically the only kind of love featured throughout the series note In the '90s, Charlie Brown had a girlfriend named Peggy Jean, and their feelings for each other were mutual. Until she broke up with him, anyways. Also, in the very early days of the strip, Patty and Shermy were a couple, but these strips are never considered canon by fans.
In Curtis, title character Curtis Wilkins only has eyes for would-be celebrity Michelle, and barely notices Chutney's crush on him.
Arms and the Man by George Bernard Shaw: Raina does start out in love with Sergei, but she's more in love with the romantic ideas she holds about what he's doing out on the battlefield, and once Bluntschli shatters her notions of romantic battle (in the first scene), we've got... Raina starting to want Bluntschli, Bluntschli thinking she's far too young to pursue, Sergei still thinking he's going to marry Raina but really more lusting toward housemaid Luca, and Luca seemingly going to marry a fellow servant. Of course, near the end, Luca pulls the rug out from under this arrangement, and everyone falls over into the right arms.
Bluntschli: 23? 23?! If you were 23 when you said those things to me, I shall take them very seriously indeed.
In Jean-Paul Sartre's Huis Clos/No Exit, Estelle wants Garcin (more specifically, she wants a warm male body, and Garcin will do); Garcin, on the other hand, is attracted to the intelligence of Ines; but Ines is fixated on Estelle, who reminds her of her former lover Florence. The fact that they're stuck in a love triangle will never be resolved is their chief torment in hell.
In The History Boys: Posner loves Dakin, but Dakin is (if not in love with, then at least attracted to) Irwin. More amusingly, Scripps has a theory that this is the defining characteristic of Christianity.
7. Umbridge has a crush on Dumbledore, who's gay. Dumbledore also seems to have a crush on Snape.
8. Bellatrix loves Voldemort, who seems to love Quirrel...
Wicked: Nessa loves Boq. Boq loves G(a)linda. G(a)linda loves Fiyero. The only requited love is between Fiyero and Elphaba, the latter of whom thought it was unrequited at first.
A Midsummer Night's Dream has several human characters falling in love with one another, then out of love and into love with someone else, all at the wrong time, thanks to the fairies using love potions to mess with their already confused relationships for the lols.
Amy Rose will try and try but she'll never get Sonic to return her feelings. However, the titular hero does like her as a friend.
A few bios state that Sonic probably doesn't dislike her as much as he appears to, hinting at a facade. But he's still 15 and she's 12 and she's very clingy. Sonic Chronicles, being a typical BioWare game, does actually allow the two to become an item if you play your cards right. Whether or not this will become Canon has yet to be determined.
Knights of the Old Republic manages to accomplish the trope backwards with the addition of the second game. Pursue romantic subplots in the first game all you want; Revan still takes off for parts unknown and never returns, leaving his love interest behind to wait indefinitely.
Also occurs with the Exile. While it's possible to have certain party members fall for you, you're never really given the option to return their feelings. At best, all your character can do is dancing around the issue. Also a male Exile can make advances on Mira, but she turns you down.
Though Revan, a prequel novel to Star Wars: The Old Republic reveals Revan (who is canonically male) and Bastila is requited after all, with Bastila being pregnant with a son sometime before Revan fought the Sith Emperor. The Exile is still ambiguous on everything, though.
Neverwinter Nights 2: Pursue the romanceable characters, you still get the Rocks Fall, Everyone Dies ending. Until Mask of the Betrayer, anyway. Though if you mod the game to restore Neeshka's romance, she is one of the characters confirmed to survive the ending...
Neverwinter Nights, for male characters: Of the three reocurring female characters, one has no interest in the player, one never progresses beyond occasional flirting, and one dies no matter what you do. Averted with Nathyrra in the Hordes of the Underdark expansion.
Though The King of Fighters mostly doesn't play this one, it does nod to it - in XI, you can get a special ending screen by playing as "Unrequited Love Team": Ramon (for Vanessa), Sie Kensou (for Athena Asamiya), and Robert Garcia (for Yuri Sakazaki).
The Robert-Yuri example comes off as strange, since their relationship was never depicted as truly one-sided in the original source (Ryo even seems to support it in AOF 3), and in KOF itself it seems that she may reciprocate... if her father Takuma actually allowed Robert to speak romantically to her. Mai-Andy remains a much more classic example (lovechild jokes and all), and in fact, in their origin series the ultimate fate of their relationship has been left a mystery, though Mai is said to be still in love with Andy but she seems to be out of luck by this point.
In regards to Athena and Kensou, they've been friends and partners ever since Psycho Soldier times, and he's very much in love with her. However, while Athena greatly cares for him as her best friend and is all but stated to know of his feelings, she simply doesn't reciprocate them. In other media, it's also implied that Athena has (or used to have) a crush on Kyo Kusanagi... whose canon girlfriend, Yuki, is a pretty close friend of Athena as well. (Besides, the girl seems to be veryMarried to the Job of being an Idol Singer, and Japanese idol singers tend to have troubleif they ever get boyfriends,,,)
Another example is that of Charlotte-Shizu-Haohmaru. Charlotte and Shizu were both in love with Haohmaru, but he is a Celibate Hero, dedicated entirely to fighting. While Shizu is depicted as being permanently interested in him and Haohmaru returns her love to a degree (he even kisses her in his SS 2 ending, though he also also warns her she's better off without him), Charlotte eventualy renounces to her feelings (Important Haircut and all) once she finds Haohmaru and Shizu together at the end of the second game.
Later, Shizu and Haohmaru seem to have set in Loved I Not Honor More. Their mutual feelings are clear, but they also know that Haohmaru's biggest priority is fighting. As for Charlotte, she also has seemingly decided to shun romance aside and live her life at the fullest.
Early on in Persona 4, Saki Konishi, the girl that Yosuke Hanamura was crushing on is killed before anything really comes of it. Later on however, he learns that Saki actually hated him and his family for putting hers out of business (Yosuke's father is the manager of a Mega Corp. supermarket that made things tougher for smaller businesses) and was only cordial to him since he's the boss' kid.
In BlazBlue: Calamity Trigger: Present time (past time had Jubei and Nine together, so that doesn't count), Bang Shishigami is pretty much all-heels to Litchi Faye-Ling, but she pretty much puts him at the 'friends' box at best; or think he's a disgusting Stalker with a Crush at worst. Litchi herself is completely dedicated to Arakune, who's... in no condition to reciprocate her feelings, and during the times he was sane, he's too absorbed in trying to upstage Kokonoe due to inferiority complex and pretty much ignores Litchi's feelings. On the other side, there's Tsubaki Yayoi who's all heels for Jin Kisaragi, but Jin just saw her as a 'good cousin' at best, because he's an asshole. And has a questionable attraction for his brother Ragna the Bloodedge, who's got other business to deal about, not even caring for Noel Vermillion's small, growing crush on him.
Cullen the Templar has quite the crush on a female Mage Warden, which ultimately ends quite disastrously. In Dragon Age II, as Knight-Captain Cullen, he implies that even by now he's still in love with her.
Carver of the second game holds a rather large crush on Merrill, but his blatant, if weak attempts to flirt with her constantly fall flat due to her distracted nature and crush on his older sibling.
This might be implied with Anders: when Hawke finishes up the 'Tranquility' quest by talking to him in his clinic, he will flirt with Hawke—male or female. If Hawke doesn't flirt back, he takes it badly and earns a handful of rivalry points. This could be his ego, or the fact that there's no nice way to let him down gently (at least early on), but...
Saints Row: The Third shows this in, of all people, The Boss, if you give her the Female Eastern European voice/personality. She often flirts with Pierce, ranging from offering to being his dominatrix to wanting to make love to him in front of a live studio audience. Pierce is...slightly creeped by the passes, maybe because he's insecure about having an affair with his boss.
In Final Fantasy VII, teenage Cloud is in love with Tifa, but Tifa barely knows he exists, which prompts him to join SOLDIER (and kicks off the story of Final Fantasy VII in the first place). When they grow up, Tifa loves Cloud, but if he knows she does, he doesn't show it, at least not until the end of the game when he reciprocates. Johnny has the hots for Tifa too, and ends up running 7th Heaven in her absence because of it. Cloud thinks he loves Aerith due to the influence of Zack's memories, but she's still hung up on Zack, and Cloud comes to his senses after her passing. Tseng loves Aerith, but while it was an open secret, he never acted on his feelings. Rude has the hots for Tifa, but every time they encounter one another she either kicks his ass or mouths off to him. Elena loves Tseng, but she's too much of a fangirl to do anything about it. Vincent loved Lucrecia to the point of madness, but she was too busy starfucking Hojo to notice him. Hojo in turn didn't care one wet napkin about Lucrecia. And Shera loved Cid so much she (accidentally) sabotaged his dream rather than let him go through with it and die (they eventually get married after Cid learns her side of the story and he names an airship after her, however).
Near the end of Brütal Legend it's implied that Lita has developed feelings for Eddie. But Eddie only has eyes for Ophelia.
Bittersweet Candy Bowl has a chronic case of this. Almost every character in the cast suffers from this trope at some point.
The webcomic Supermegatopia features Grease Monkey in love with Weasel Boy, Weasel Boy in love with Tiger Lass, Tiger Lass in love with Mighty Yak, and Mighty Yak is too dumb to notice the Love Dodecahedron. More links on the chain get added over time.
In Sluggy Freelance Zoe cycles through this trope pretty darn quickly in this strip. Before she'd always rejected Sam's advances, finding him creepy. Then, when he goes missing, she laments how she let her "last hope of true love" slip through her fingers. Then, when Sam turns up alive and well, she's back to thinking he's disgusting. And then, no more thirty seconds later, she finds out Sam's gotten married. Instantly she's calling him her one true love again.
The gods in A Momentof Peace form a love conga line. The god of Trust and Betrayal loves the goddess of Stories, who loves the god of In-Between Places, who loves the goddess of Glory.
In All Roses Have Thorns, Galvan is in love with Auron, but Auron very obviously still has feelings for Collamair, and Collamair is in love with Faylin, but Faylin was in love with Isleen, but she rejected him. Wow.
Davey of Maggot Boy is hopelessly in love with Sam, which will never go anywhere because he's dead, and she has a huge crush on her boss Lazaro, but he just hates everyone in general.
Homestuck: Okay. Karkat had a confirmed one-sided crush on Jade, as did Tavros, and she rejected both. Gamzee at one point had a flushed crush on Tavros, and it's unknown how he responded. Vriska also had a very one-sided crush on Tavros and also had a crush on John that he seemed to return at first but has since grown out of. Kanaya had a one-sided crush on Vriska. Karkat seems to still have feelings for his ex-girlfriend Terezi. Nepeta had a one-sided crush on Karkat, while Eridan was also after Nepeta for a while, as well as half the cast, but most dramatically a red crush on Feferi and a black crush on Sollux. Both Sollux and Equius had things for Aradia which were complicated after she died. After being brought back to lifeas a robot, Aradia seemed to retain some feelings for Sollux, at least to the point where her last action before randomly causing herself to explode was to hug him, though at that point he was dating Feferi. Both WV and AR had things for PM which she never found out about before AR died and WV was, at the least, gravely injured. PM has a black crush on Bec Noir, who has a red crush on her. Finally, all of the Alpha kids have a crush on Jake, plus Roxy also has an unrequited thing for Dirk, while Jake wouldn't have minded the idea of going out with Jane or Dirk, and tried to date Dirk, but isn't really that interested, and also has a crush on Aranea, who seems to have a pretty similar lukewarm interest in him.
Happens between minions in Girl Genius. Violetta and Snaug both crush on Moloch, who goes after Sanaa instead, who finds it stupid. However, things seem to change in favor of the Moloch-Snaug pairing.
In Sandra and Woo the Alpha Bitch Zoey is revealed to be a lesbian and has a crush on her friend Michelle. However, Michelle is shown as being homophobic and doesn't return her feelings after she kissed her. Cue pity.
The Frederick, Leif and Canta triangle in Sire. Frederick loves Canta who loves Leif who is withdrawn and closed off to everyone. It should be noted that as Lineage Children of Raoul, Erik and Christine this alters their fated narrative a little bit.
Family Guy: Played as a recurring gag with Meg. Virtually every boy she attempts to ask out rejects her, all of them making up wild excuses to get out of having to go out with the homely Meg.
Camp Lazlo: Patsy loves Lazlo, who may or may not love her. Clam loves Gretchen who loves Edward who hates her. Lumpus loves Jane loves Pothole loves Lumpus, until Jane gets engaged to Manfred. That last one is a source of much frustration and humor on the series, until on the final season Lumpus proposes to Jane.
The only really consistent, reciprocal romance on the show (other than Arnold's grandparents) seems to be the Ship Tease between Gerald and Phoebe, and even they've had occasional other Love Interests. There was also Harold and Big Patty, the two Jerk with a Heart of Gold former bullies, who actually seemed to be dating. It was kind of surprising, given Harold's immaturity. (And the fact that he was formerly Ship Teased with Rhonda.)
The Fairly Oddparents: Timmy loves Trixie who ignores him. Tootie and Veronica love Timmy, who doesn't like them. Juandissimo loves Wanda, who is spoken for. Vicky loved Gah, who didn't love her, Ricky, who only loved her money, and Winston, who was too shallow. Mark loves Vicky, who isn't interested, and Mandie wants to marry Mark, but he's terrified of her. Only couples who were already established as dating or married ever get love.
Butch Hartman himself has stated that any of Timmy's potential love interests could be the mother of his future children (seen in a flash-forward scene of "Channel Chasers"), although Trixie and Tootie are the most probable candidates.
Most of Clone High has Joan in love with Abe who is in love with Cleo who eventually dates him but was in between him and JFK for awhile (who still holds a torch for her while she dates Abe). The finale features Abe realizing he loves Joan, leaving Cleo unrequited, only to find her in bed with JFK, who has just admitted to liking her. Meanwhile in the beta storyline Marie likes Gandhi who doesn't realize it until it's to late and she's dating someone else. Unlike the main story though they actually do hook up.
Danny Phantom: Sam has had a long-standing crush on Danny, who bounces back and forth between not noticing and returning her feelings. Danny, in turn, likes Paulina, who in some episodes seems to be dating Dash and most definitely has a crush (to an almost creepy extent) on Danny Phantom (the alter ego). He also likes Valerie, but they never really get together. Tucker, on the other hand, just likes girls. Oh, and Vlad Masters is in love with Maddie Fenton, who is married to Jack, and thus Vlad often plans to kill Jack so he can have her. An Unrequited Love Switcheroo also happens in a few episodes: for example, in one episode, Dash is hitting on Jazz, but this is never toyed with again; however in another one, Jazz has a dream in which she is married to Dash.
Pretty much the entire plot of "Simple Ways." Spike has had a crush on Rarity since the first episode of the series. Rarity, it turns out, has a Stalker Shrine to Trenderhoof in her boutique. When Trenderhoof visits Ponyville, he is oblivious to Rarity's overtures, but is instantly smitten with Applejack. And Applejack, who has a lot of work to do, wishes Trenderhoof would just leave her alone. Summed up nicely in this picture.
Rarity is aware of Spike's feelings and as we find out in "Secret of My Excess," is flattered, but doesn't return them. She was once interested in Prince Blueblood, until she found out that his only love is himself.
In Jackie Chan Adventures season 2, when the Sky Demon changes shape and takes the role of a student at Jade's school to gain her trust, infiltrate the shop, and retrieve his lost tail (It Makes Sense in Context), Jade is heavily implied (and confirmed in a later episode) to have a childish crush on him, and is genuinely hurt when his identity is revealed.
In Superjail!, the Warden loves Alice, who loves an inmate she calls "Sweet Cheeks". Meanwhile, Warden shared some Belligerent Sexual Tension with the Mistress, although she was disgusted at the idea of having slept with him and a later attempt went sour when Warden had no clue what to do with condoms. Meanwhile, Mistress dated Lord Stingray for a while before breaking up with him, and then wound up sleeping with Alice, who only had sex with her to loosen her up and get the job done. Alice had also killed Stingray's wife, Mistress Kilda, who was stated to have a boyfriend on the side (Catastro).
Jared was once shown to possibly have some sort of repressed feelings for the Warden in a dream of his, but the Warden (who saw into the dream) became disgusted at the sight of Jared making out with him.
Jared and Charise are about the only stable ones, though they are unable to actually get together due to the distance between the prisons. They attempted to have Warden and Mistress get together for the sole purpose of THEM being able to stay close to each other, although their plan failed (see above).
The only other couple that is stable (somewhat) would be Jean and Paul, the "Gay Inmates" duo.
In other examples of unrequited love, there's the subject of Alice's backstory involving her having feelings for her old warden (Mr. Toddly), who turned out to be a gay man and who fired her when he discovered she was undergoing a sex change. The Warden also had one-time feelings for the alien woman Hunter who's actually a shape-shifting dog, who had no interest in him other than trying to find out if he knew the Twins' whereabouts.
The only two "couples" first introduced in Teenage Mutant Ninja Turtles (2012). Donnie likes April, but she shows little to no interest in him, even when he's expressing his feelings for her. Leo had an interest in Karai, Splinter's supposedly dead daughter, but she didn't feel anything for him in return. Other than the Interspecies Romance between these couples, they wouldn't work out since Karai hates Leo for attempting to kill her adoptive father, the Shredder, and April hates Donnie and his brothers for accidentally mutating her father. That and the fact that she's seeing Casey, who eventually showed up. While April did eventually forgive them, by then she and Casey were already friends and growing close during her time away.
Early episodes of The Muppet Show, as well as Muppet Babies, play this as a Running Gag. In the former, Adult Gonzo gets over his crush on Piggy when he meets Camilla. In the latter, a younger Gonzo has an hopeless crush on Piggy, who only has eyes for Kermit. But Kermit has no stomach for this whole love thing at all.
Ed, Edd n Eddy: The Kanker sisters like the title characters, but all three of them are smitten with Nazz, who has a crush on Kevin, who cares more about his bike (though throughout the show Kevin is clearly attracted to Nazz as well). In one episode Jimmy is jealous because Sarah falls in love with Edd, but Edd is just scared that she won't leave him alone.
While (thankfully) not always the case, this trope is often true of Real Life. As a rule of thumb, people tend to be most attracted to people who are "out of their league": more attractive or talented or popular than themselves. As a result, almost everyone has had a lot more experience with unrequited crushes, both as the crusher and the crushee, than with actual successful relationships.