Finn and Raven from The 100 have this in their backstory together. They've been so close for so long, they actually refer to each other as family, even after they've broken up.
The 2000 TV version of Arabian Nights makes it that Scheherezade and the Sultan were childhood playmates, and she's not marrying him only as a noble act of self-sacrifice. By the end, he (comes to realise that he) loves her too.
Boy Meets World: Cory and Topanga met in elementary school, become friends, and then become so oblivious to their mutual attraction that their friends set them up.
The comics reveal Dawn and Xander, who appear to be together in the comics now. Dawn has always had a crush on Xander.
Also opens up some questions about free will when you realize Dawn was a construct built by the monks to hide the key. ALWAYS in her case is literally from the beginning of season 5 when she was made. The crush memories she has that ARE shown from before then were implanted by the monks.
Diego and Temo were friends since childhood, and even dated briefly before the start of El corazón nunca se equivoca. Unfortunately for Diego, Temo was too in love with Ari and couldn't reciprocate.
Adam to Toby in Dante's Cove — by the time he realises he's gay and in love with his best friend, Toby is in a committed relationship with Kevin.
Amy and Rory, from the Eleventh Doctor's era... grow up as childhood best friends, with Rory having a crush on Amy. When Amy meets the Doctor for the third time, its revealed Amy is engaged to Rory, and Rory soon joins the Doctor on his adventures alongside Amy, until Rory is erased from time, comes back to life as an Ancient Roman, and then is revealed to be a disguised Auton created from Amy's memories. Oh dear. But then he comes back to life for real, and they get married at the end of the 2010 series. For bonus points, the one who makes Amy realise Rory has feelings for her in the first place is their daughter.
"Demons of the Punjab": Umbreen and Prem grew up together as neighbours, before they got married. Sadly, it ends in tragedy.
Emergency!: Roy and Joanne Desoto. One of the few things we learn about them is that they were childhood friends who later fell in love.
The Flash (2014): Barry is in love with Iris when they're kids - "Before I knew what the word love meant" - and is highly embarrassed when he has to move in with the girl he has a crush on. Iris, for her part, takes another decade to realize her romantic feelings for Barry.
Olivia and Peter in Fringe, though (like the Simpsons example below) it's an interesting case. They briefly met as children while both trying to run away from their guardians, and later got together as adults, but neither of them remember their first meeting as a result of suppressing traumatic memories from around that time.
Barry Goldberg and Lainey Lewis. She has been Erica's friend since they were kids and got to know Barry just as well. Though it always seemed to be that Barry's crush was one-sided, Lainey eventually reciprocates in Season 2.
Erica Goldberg and Geoff Schwartz. Barry's good friend who also got to know Erica when they were kids. Once again, was one-sided on Geoff's part, but then after a season of obsessing over Erica and constantly being shut down by her, ANOTHER of him dating another girl with Erica discovering her own feelings and trying to win him over them, them getting together only for a dickish plot twist to push them apart again and one final exhausting plot line involving the prom episode, it finally FINALLY came to fruition much to the relief of the fans!
Lilly and Oliver in Hannah Montana. They have been best friends since preschool and eventually get together.
Harper's Island. Henry desperately loves his childhood best friend Abby, not realising he was made to spend so much time with her in childhood because he was adopted and she is actually his half-sister.
How I Met Your Mother: The "second-greatest love story" Ted has ever heard involves a girl who is explicitly described as a Girl Next Door and her childhood friend. To elaborate, the girl in question is almost never single (since she's the perfect Girl Next Door hottie) so there's a limited "window" where she's single. Long story short, the window opens and Ted tries to keep other men away from her. She manages to get away from him and his rival, and on her way home meets the man she had grown up with and loved (mutually). The window never opened again.
iCarly: Freddie and Carly have known each other since the sixth grade. Freddie started off on the show with a crush on Carly. He saves her life, they get together for an episode but break up.
Mary Connell and Danny McCoy in Las Vegas, both having known each other since they were toddlers and Danny always coming to Mary's aid when her molester father would come after her. They come together and he proposes to her in the first season finale, but they end up breaking it off because of Danny's post-traumatic stress disorder. Two seasons later, Danny winds up falling for Delinda Deline (whom he had a brief affair with in the first season) and they become official, even impregnating her later on, much to Mary's dismay.
In Lost, Kate was Tom's childhood sweetheart, but then he was married.
Maddigan's Quest: Boomer to Garland. At a crucial moment, Garland chooses to trust quasi-love interest Timon over him— which turns out to be a mistake.
The Middle: Sue Heck and Sean Donahue knew each other ever since they were little kids (for about as long as Sean has know and been best friends with Sues brother Axl) and while shed always had a crush on him for the longest time, hed never seemed to have feelings for her until years later. Though those three times that he attempted to escort her to a dance, prom or ball were likely him testing the waters for whether or not something might actually be there.
Interesting case on Mighty Morphin' Power Rangers with Kimberly and Skull. Skull maintained a strong and obvious crush on Kim, including flash backs to their childhood. His affection was returned on several episodes before Kim's departure and he might have stood a chance if not for her boyfriend at the time. However, writers have stated that they had intended for Kim to be the mother of Skull's confirmed son, Spike, in later seasons.
Freddie: I love her, Tommy. I've loved her since she was 9 and I was 12. And she loves me the same.
Charlie is Megan's childhood friend who has a crush on her in Privileged.
Pushing Daisies: Chuck and Ned grew up next door to each other, but were separated when Ned went to boarding school and Chuck had to move to her aunts' place. They reunite as adults (after Ned revives Chuck), and promptly get together.
Queer as Folk UK: Vince finally rides off into the sunset with Stuart at the end of the story. The US version has the failed version— Michael is in love with Brian, who loves him but not in that way. Both eventually find love with others and move into Heterosexual Life-Partners territory (minus the heterosexual part).
On Revenge, Jack Porter and Amanda Clarke were Childhood Friends and were so close that he continues to take care of her dog for her after 18 years and named his boat after her. Now he has feelings for Emily, not realizing that she is in fact Amanda all grown up. Emily/Amanda returns the feelings but doesn't act on them due to her revenge plot. Also she may or may not be falling for her Honey Trap victim Daniel.
Things get even more complicated when the real Emily Thorne, who of course has taken on the identity of Amanda Clarke, shows up and Jack ends up marrying fake-Amanda.
If the Series Fauxnale of Scrubs is any indication, J.D.'s son Sam and Turk's daughter Izzy will become this! It's unclear if this is a vision of things to come or just J.D.'s final Imagine Spot.
From Japanese drama Shōkōjo Seira, Kaito's childhood friend Yukari grew increasingly jealous of his relationship with Seira. It didn't help that Kaito romantically pursued Seira (who reciprocated those feelings) and his family immediately accepted Seira into the family while being critical of Yukari's attempts to fit in (it's implied she was only nice to Kaito's family when Kaito was around). She would eventually give up Kaito to Seira.
Clark and Lana had been classmates since childhood, with Clark harboring a crush on her. Eventually, they got together. Clark finally grows out of it during the last couple seasons.
Chloe Sullivan was one of Clark's closest friends since the eighth grade and has a crush on him, though she eventually matures out of it, similar to how Clark himself grew out of his former crush on Lana.
Switched at Birth: Emmett's been waiting for his long-time best friend Daphne for eight years, but she's only just realized she's into him too...right after he starts dating Bay. Whoops.
On Teen Wolf, Stiles' childhood friend Heather resolved to lose her virginity on her 17th birthday, and Stiles was her intended target. Subverted somewhat in that it is not indicated that she was in love with him, but felt comfortable enough with him that he was the preferred choice for her first time.
The Twilight Zone (1959): Deconstructed in "Spur of the Moment". Anne Henderson and David Mitchell were childhood sweethearts who always knew that they would get married when they were older. They were engaged but Anne broke it off and became engaged to the investment banker Robert Blake. However, she has a change of heart and elopes with David on June 13, 1939, in the middle of her engagement party. This proves to be the worst decision of her life as David becomes an abusive alcoholic who bankrupts her family's estate and turns her into an alcoholic as well.
This was the original series arc for Wizards of Waverly Place. The main character Julia (Alex) was a young wizard whose best friend was Jordan (Justin), the "Boy Next Door" — and the idea was for them to have wacky adventures while slowly but surely becoming something more as they grew up. (Selena Gomez and David Henrie were hired for, among other things, the obvious chemistry they had as best friends who would grow into a romantic relationship.) However, the chemistry and timing that the two had as potential love interests didn't go away when Executive Meddling turned them into siblings, creating a Relationship Writing Fumble.
Kevin and Winnie in The Wonder Years is this and a less tragic version of Star-Crossed Lovers; they clearly loved each other, but the ending narration reveals that Winnie went off to Europe and Kevin married and had a son with another woman.