Zhou/Wicked from ½ Prince spent 8 years in love with Lan/Prince, but he lived far away and she never recognized him when he played online with her. When she starts on a new game at the beginning of the story she develops thanks to a bet made with her brother, and ends up a much more self-sufficient person than he's used to. Her new personality makes his attempts to coddle and look after her like he always has only offend her, and she ends up choosing his love rival Gui.
Brennus: According to flashbacks, before Lady Light and The Dark became the leaders of the superhero and supervillain communities, they were this, having been born in the same room on the same night (through sheer coincidence mind you), and spending their childhoods doing everything from catching serial killers to fighting in World War I together, to the point where it's acknowledged that getting married would be simply a formality. Doesn't stop him from being unbelievably nervous about asking. Of course then Point Zero happened and they got their powers. Their relationship since then has been...complicated, to saythe least.
Wuthering Heights: Heathcliff and Catherine have a passionate, enduring love, were raised as brother and sister, and may be half-siblings (though this last element remains ambiguous), and Incest Is Relative.
Young Amy ultimately marries Theodore "Laurie" Laurence who she's known since childhood. When she was afraid she would die without being kissed, he promised to kiss her before she died. She surely considered herself victorious when he started expressing interest in her.
Laurie himself experiences the non-mutual form of this trope with Amy's older sister Jo. He loved Jo since they met as teenagers, but she only ever saw him as a brother/friend. When he finally has to say it straight out for her to accept it (with Jo ignoring all his advances and moving away to avoid the inevitable), she immediately shoots him down and he freaks. He gets better later, enough for him to fall for the now grown-up Amy.
Nat Blake and Daisy Brooke (Laurie's niece), who had been good friends since Little Women's sequel Little Men, got married in the final sequel Jo's Boys.
In the last sequel, Jo's Boys, Tommy misses out on Annie aka Nan since she just wants to be a single doctor, taking care of people. He's not too broken up, though; he marries Dora, a girl he at first only got with to make Nan jealous when she's his perfect match and Nan couldn't care less. In fact, Nan was all "... the Hell?" when Tommy reminded her of their Childhood Marriage Promise...
There are several examples in the Anne of Green Gables series. Perhaps the most extreme is with Miss Lavendar and Stephen Irving from Anne of Avonlea: they met when Lavendar was six and Stephen was nine, and soon after made a Childhood Marriage Promise. They were formally engaged when Lavendar was twenty... and then had a fight and broke up. Stephen then moves to America, marries, and fathers a son, Paul. Paul's mother dies, and Paul moves back to Avonlea to live with his grandmother, befriends Lavendar (who remained single for twenty-five years, having regretted their break-up and not wanting to marry anyone who wasn't Stephen), and writes to his father about it. Stephen returns to Avonlea and reconciles with Lavendar, and the two are married... twenty-five years after they were engaged and almost forty years after they had met.
Anne and Gilbert themselves, who were friends after she stopped hating him.
Most of Anne and Gilbert's children, as well, marry childhood friends. The most notable is their youngest daughter, Rilla, who marries her childhood friend/crush Ken Ford. In Rilla of Ingleside, Ken gives Rilla her first kiss before heading off to war and begs her not kiss anyone else while he is gone. The book ends with his return and a sweet proposal using her childhood nickname.
Averted with Rilla and Carl. They were great childhood friends because they were close to the same age, but they mutually agreed to never get together.
They used to talk together of almost everything and were teased about each other at school; but one evening when they were about ten years of age they had solemnly promised, by the old spring in Rainbow Valley, that they would never marry each other. Alice Clow had "crossed out" their names on her slate in school that day, and it came out that "both married." They did not like the idea at all, hence the mutual vow in Rainbow Valley. There was nothing like an ounce of prevention.
In Frankenstein, Victor and Elizabeth were raised together because of her father's tragic fate and his mother decided to invoke this trope. Tragically, their marriage was short-lived.
The Chronicles of Prydain: Taran and Eilonwy. When Taran finally asks her to marry him after years of silent pining, Eilonwy replies, "Of course I will, and if you'd ever given any thought to the question you'd have already known the answer."
Dawn by V. C. Andrews : Dawn and Jimmy were raised as brother and sister, but after the truth is revealed, Jimmy admits that he used to wish Dawn wasn't his sister because she's the only girl he's ever wanted. She eventually gets over this revelation and they get married.
In the Dragonlance novels, Laurana and Tanis are childhood sweethearts, complete with a Childhood Marriage Promise. Tanis then leaves Laurana after being confronted by her family who do not believe a bastard half-human like him is good enough for an elven princess. Many years later Tanis is reunited with Laurana who is still very much in love with him and has become an incredibly beautiful woman. Tanis soon realizes he still has feelings for her as well, but the situation is complicated because he is now also in love with another woman, Dark Action Girl Kitiara Uth Matar, setting up a three-book-long Betty and Veronica triangle. It is only after Tanis sees Laurana's beauty and courage while she is a prisoner of Kitiara that he realizes she is the one he truly loves. Tanis then saves Laurana from a Fate Worse Than Death and the two are married.
Severus Snape was in love with his childhood friend Lily Evans, as revealed in Harry Potter and the Deathly Hallows (and hinted at since Order of the Phoenix). In fact, his unrequited love for Lily, and guilt over her death, was a major factor in Voldemort's downfall — not bad for the maladjusted kid who fucked up so badly with the girl he liked that he got thrown over for an ex-Jerk Jock.Snape's case also provides an interesting look at the implications of this trope in regards to the "one-sided —> care for child or sibling" idea. Namely, Snape cared for Harry because he was Lily's son, but was also cold and cruel to Harry because he was James's son. Funny how things change when I Want My Beloved to Be Happyisn't in play.
At the same time, this trope is fairly common in the universe of Harry Potter, as not only do wizarding children go to the same school as others in their region/country from age 11 on, but also share a "house" for seven years. It's a dorm room atmosphere with the same faces from adolescence through puberty to of-age young adults.
Nancy Werlin's Impossible. Lucy and Zach are kind of complicated by a prophecy that Lucy, like her mother, will go insane when she gives birth to a child if she doesn't complete threeimpossible tasks, but they work it out in the end.
The Sorrows of Young Werther: Although the length of their relationship is not specified, Lotte and Albert have been close friends long enough that he was in the room with her when her mother died. They remain an established couple throughout the novel, much to the despair of her admirer , Werther, who blows his brains out.
Will and Alyss in Ranger's Apprentice. To the Genre Savvy readers, it was obvious there were some feelings between them even as early as the start of the first book.
In P. G. Wodehouse's Jill the Reckless, Wally. He confesses that his bad behavior stemmed from a crush in childhood.
Twilight: Jacob Black spent some time with Bella when they were kids and tried to re-connect with her when she moves to Forks, but loses out to Edward.
Due to Applied Phlebotinum, the "care for child or sibling" aspect is played squickily straight as it turns out that Jacob's tribe is destined to "imprint" on their fated mates upon meeting them, even if they have to undergo a ridiculously long stretch of Wife Husbandry before they can marry them, and it turns out that Bella and Edward's daughter was Jacob's all along—but don't worry, because she's half-vampire, she ages ridiculously quickly until reaching adulthood (biologically 17-ish when chronologically 7), at which point she stops aging like a normal vampire. And Jacob "imprinted" on her while she was still an unborn fetus, though he didn't realize it until she was born and mistook it for his feelings for Bella (which may or may not have actually been caused by the fact that she was fated to be his mother-in-law.)
Speaker for the Dead: Miro and Ouanda with a twist: After he and Ouanda turn out to be half-siblings, he gets sent out to space, and because of relativity, everyone else ages a few decades. So while Ouanda has aged 30 years, long since forgotten Miro, and married someone else, Miro still misses her and feels abandoned. Doesn't help that he's handicapped either.
Littlefinger and Lysa Arryn. Whom he marries — and then kills. She was batshit insane and fond of throwing people off tall mountains, so it's not like she didn't have it coming... though most of his other actions have less legitimate excuses.
Rand and Egwene in The Wheel of Time series are more of a subversion. The decision to break up is mutual, they remain friends, and they both get happily involved with other people later in the series.
Cinderpelt and Firestar Warrior Cats (verified by Word of God). She was originally Firestar's apprentice, meaning it started out as a case of Hot For Teacher, but by the end of the first series, she was one of his closest friends. Firestar however is mates with another she-cat.
Mason to Rose in Frostbite. He was her friend since childhood, was crazy about her, and was boyfriend material. Emphasis on the unlucky. He was both a Hopeless Suitor and an early Strigoi casualty.
Sylvie, the heroine of Kiss by Jacqueline Wilson, believes she is in love with her friend Carl and has always imagined they would get married one day — only for him to come out as gay. It's left somewhat ambiguous at the end of the book when Carl kisses her and tells her he will always love her; but she appears to take this in the platonic sense.
Zigzagged in Miranda July's Something That Needs Nothing. The main character is in love with her best friend, Pip, and has been in love since they were children. The two have had very sporadic sexual encounters, but the main character is resigned to the fact that Pip will never love her romantically. Once she starts working as a peep show artist, and devises a new persona for herself, that's when she gets Pip to fall for her.
Orrec and Gry in Annals of the Western Shore. Their families are allies, and they always considered themselves obligated to each other even if their parents were discussing marriage prospects. In the second and third book, they're Happily Married.
After 20 years of friendship (and thinking the other was straight), Mortimer and Gabriel finally get together in Demon Meat For Dinner.
Cryl Durav from the Kharkanas Trilogy is of the unlucky sort. He and Enesdia grew up together, and Cryl is very much aware of his feelings towards her. Enesdia, on the other hand, thinks of him as just a second brother, although what her behaviour says is at odds with this. It ends tragically with both of them dead at Enesdia's wedding.
Happens in Halo, of all places. In the story Palace Hotel in Halo: Evolutions, it's revealed that John-117, when he was a child, had a female friend with whom he made a "silly Childhood Marriage Promise". A month later, John was drafted for the SPARTAN-II project. During the beginning of the Battle of New Mombasa, John found her again, still holding a photo of him, and realized that she had missed him... but knowing well that she could die in the next fifteen minutes in the coming skirmish, and that revealing his identity to her would compromise the secret of the origins of the SPARTAN-II soldiers, he chose to stay silent, leaving the poor woman to think he had died when he had been a kid, when the Spartan in front of her was him.
In Addicted, Loren Hale and Lily Calloway were best friends since childhood and began a fake relationship as teenagers. In college, they began dating for real and ended up married a few years later.
Lusa from Seeker Bears befriends a male black bear named Miki in Great Bear Lake. He appears again later and as adults they become mates.