Clarke and Lexa have this in Season 2, eventually leading to a kiss in "Bodyguard of Lies", but any potential relationship is but on hold due to a) Clarke still recovering from killing her previous Love Interest, and b)Lexa betraying their political alliance shortly afterwards.
Jack and his mother-in-law, Diana (who's around his age). Definitely not as squicky as it sounds. They began working together to bring his wife home and are highly attracted to each other.
Angel: Angel and Cordelia and Wesley and Fred absolutely torture the audience with this for years. Both cases are finally resolved in the final season with the deaths of both women. Cordelia and Angel never get to have sex, and it's unclear if Fred and Wesley ever do (she has a line while she's dying about "finally" getting him up to her bedroom, but they might have slept together at his place). This could fit with all the A.I deaths on this show. Doyle died right after kissing Cordelia, after pining after her for his entire time on the show, Fred died only one episode after finally getting together with Wesley, and Wesley died right as he was beginning to get along with Illyria.
Oliver and Felicity remained at a smolder for two seasons. Oliver clearly wants her, as evidenced by his obvious jealousy when Felicity shows interest in Barry Allen, but never makes a move, variously pursuing Laurel and Sara Lance instead.
Oliver Queen and Laurel Lance had this at the beginning of the series, and even after Oliver got together with Felicity, hints of it still lingered. Season Five eventually confirmed that Oliver was still in love with Laurel even during his relationship with Felicity (with the crossover basically confirming that Laurel was the love of his life), and always would be — something that only provides more grief for Oliver, as Laurel was Killed Off for Real in Season Four, after confessing that she still loved him as well. The love he still has for his Laurel is part of why he is so conflicted over Black Siren, Laurel's evil Earth-2 doppelganger, who, despite all her malice, is still in love with her Oliver, someone that is also deceased.
Speaking of Barry, he and Iris West are fraught with UST. Barry has been in love with Iris for almost all his life, and despite Iris' adamant belief otherwise, it's clear to everyone, including her Season One boyfriend Eddie Thawne, that she feels the same. It takes a while, but they finally start dating at the start of Season Three.
Caitlin Snow had this with "Jay Garrick" in Season Two, after her husband Ronnie Raymond died in the Season One finale. They kissed at Christmas and were vaguely implied to be dating afterwards until "Jay" died. Then, it turned out "Jay" was actually Hunter Zolomon aka Zoom, Big Bad of Season Two. Once Caitlin learned that, she wanted nothing to do with him. Zolomon tried to get her to love him again by kidnapping her and trying to invoke Stockholm Syndrome, but after she rejected and abandoned him to return to Team Flash, he gave up on that and treated her as another enemy.
Sara Lance and Leonard Snart had some low key UST during Season One. Unfortunately, it remained mostly unresolved aside from a kiss shared right before Snart made his Heroic Sacrifice in the penultimate episode.
Amaya Jiwe and Mick Rory in Season Two, though this seems to be mostly scrapped in favor of an Odd Friendship. Amaya also had some with fellow newcomer Nate Heywood — they got together later in the season.
Kara Danvers and James Olsen during Season One. They broke it off at the beginning of Season Two after realizing they were Better as Friends.
Kara Danvers and Mon-El's span the first half of Season Two before they finally get together. An argument could be made that it started off as Belligerent Sexual Tension, seeing as their relationship didn't exactly start off on the right foot, being from sister planets that hated each other and Kara believing, at one point, that he attacked the President.
Ashes to Ashes: Gene and Alex have been dancing around the will-they-won't-they issue since the first episode of the show, the whole police force already thinks they're shagging, and the writers have ramped up the tension even more drastically in the second series. Of course, any potential relationship between the two may be seriously hampered by the fact that Alex is quite probably trapped in her own head and Gene himself is quite possibly imaginary... Pretty darn resolved as of the series finale: There's feelings there, but Gene, as the guardian of Dead Copper Purgatory, can never move on when there's new coppers to help, and it's time for Alex to "cross over". They do kiss before Alex walks into the Railway Arms.
The Avengers: John Steed and Emma Peel. And episodes when he's there when she wakes up! Patrick Macnee once said, "Of course they're sleeping together! It doesn't mean they have to show the world!" Cue PSL.
Steed: All this time I've known you, and I never knew you could sew! Emma: Well, our relationship hasn't been exactly domestic, has it?
Steed also had reams of UST with Peel's predecessor, Cathy Gale. And after Peel left her successor, Tara King, was often shown sharing romantic overtures with Steed.
Battlestar Galactica (2003): Starbuck and Apollo have unresolved sexual tension even after having sex. There was a whole episode with an A-plot about it, aptly named "Unfinished Business".
The Big Breakfast: The '90s breakfast show was powered by UST when Johnny Vaugn and Denise Van Outen were the two co-hosts. Even the advertising trails hinted at it when they were rehired.
Bones: Everywhere. The season four finale shows how Brennan and Booth would be if they got married — it's really boring without the UST. Word of God states that Booth remembers the fantasy. Booth's present feelings might be the result of brain damage and everyone from Angela's psychic to his own Badass Grandpa is telling him to hook up with Brennan. Episode 100 reveals that they were attracted to each other a year before the series started but then had a very bad falling out. Back in the present they finally open up to each other on camera, but Bones tearfully admits she can't get rid of her doubts about relationships and Booth is tired of waiting for her to change. Resolved off-screen.
Buffy and Angel are the staple for this trope. They have it in most — if not all — of their interactions since they Can't Have Sex, Ever or else Angel turns evil or the world ends (literally). The entirety of Season 3 was just Bangel UST climaxing in a single orgasmic Kiss of the Vampire.
Buffy and Spike in the latter part of the series. It was officially resolved in S6, then back again throughout all of S7 and continued in the comic continuation until it's finally resolved in S10.
There was sexual tension between Buffy and Xander, especially in the second half of season two, although it largely went away as the series went on.
Buffy and Faith. The Les Yay between the two was through the roof, especially in Who Are You?, where Faith-in-Buffy's body takes a bath as the first thing that she does after the switch.
The Cape: Has some undercurrents of sexual tension between Vince and Orwell, but it doesn't come out very often.
Castle: This is the entire point, which has been described as "Moonlighting meets Murder She Wrote". More belligerent in the first season, but after some Character Development settled into a serious case of this. It doesn't help that Castle's a flirt and Beckett's a tease. Resolved as of the Season 4 finale.
Cheers: Had UST as a strong teaser between Sam and Diane. Resolved, but repeated. and averted.
Chuck: Virtually every semi-regular female character except his sister has this with Chuck. In the first season, people thought there was even UST with her. And there was. Chuck was originally going to start the series with a girlfriend, and when that was scrapped many of her lines were given to his sister...resulting in a sister who's somewhat creepily concerned with her adult brother's love life.
The study group complains about this getting unbearable in the episode Modern Warfare:
Abed: To be blunt, Jeff and Britta is no Ross and Rachel. Your sexual tension and lack of chemistry are putting us all on edge. Which is why, ironically - and hear this on every level - you're keeping us from beingFriends. Britta: Jeff and I do not have sexual tension. We just argue all the time. Shirley:[smiling] Awww, just like Sam and Diane! [stops smiling] I hated Sam and Diane.
Beyond that example, Jeff and Annie have been smouldering with each other since season 1 and kissed at least three times. The show has demonstrated various times that no matter how much they try to deny it, they always end up close, pairing up for campus-wide games and getting jealous of any attention they get from other people. In the season 5 finale, Jeff's feelings for Annie are the key to opening the door and saving Greendale. In the season 6 finale, they admit that while it's not the best time to get together—they both have some issues they should work out on their own and Annie is leaving for an FBI internship—they admit they have feelings for each other and imply they'll get together in the future.
Annie:I think if you don't kiss me goodbye you'll regret it for the rest of your life.
Cranford: The tension between Laurentia Galindo and Mr Carter smokes up the screen. She's a Baronet's daughter from an impoverished family and earns her living as a milliner, but still is on visiting terms with Lady Ludlow. He's Lady Ludlow's land agent. It stays unresolved because Anyone Can Die on this show.
Grissom and Sara. Still present even though they've gotten together, although now it's more like "Repressed Sexual Tension" since two members of the same shift aren't supposed to date. Plus, Grissom's her supervisor. Again when Sara leaves, then returns as a guest during season 9 with an undertone of the relationship maybe falling apart. Ultimately Sealed with a Kiss when William Peterson followed Jorja Fox in leaving the show - only the second kiss they ever have on-screen. The first was in Goodbye and Good Luck.
When Sara's back and Grissom's gone, lab rats Hodges and Wendy became the main UST couple lampshaded in a Season 9 episode when one of their co-workers says what the rest are thinking and asks why they don't just admit to each other how they feel. They do, in the "Take That!, Darker and EdgierRemakes!" episode. Hodges then transfers to another shift to ensure they don't get fired. Wendy finally plants a big one on Hodges in the "Field Mice" episode. Whether or not it means they will be together remains to be seen. However, then Wendy leaves the series.
UST is present with Warrick and Catherine.
dads: The series ends (abruptly, due to cancellation) with Veronica getting engaged, causing Seth Green's Eli to lament that he didn't act sooner.
Daredevil (2015): Matt Murdock and Karen Page right from the start have some visible UST, but it's clear Matt's unwilling to act on it because he's hesitant about her finding out about his alter ego as the Devil of Hell's Kitchen. By season 2, they've loosened up enough that they finally have their first kiss and first date
Dark Angel: Max and Logan are in a perpetual state of "string ready to snap" UST. The show hinged on it so much that the writers infected Max with a retrovirus genetically targeted to kill Logan when they have skin-to-skin contact.
Degrassi: Plays with this whenever a pairing is not put into effect immediately. Clare and Eli's UST is certainly the most recent.
The Dick Van Dyke Show: Very evident in the flashback to the night Rob and Laura met for the first time, and the flashback to the night Rob nervously proposed and Laura nervously accepted, and one more flashback to when they were still dating. Obviously resolved since they're married in every other episode, and have a son.
Liz. In particular, he clearly flirts with her in two scenes in "Spearhead From Space", though it is only to trick her into doing what he wants.
Jo. There's some pretty heartwrenching scenes of the Doctor manfully agonising inside because he knows he can never pursue her.
The Fourth Doctor and:
Sarah Jane. This could have been intentional, or could have been real-life sexual tension accidentally manifesting onscreen (both Tom Baker and Elisabeth Sladen have admitted they fancied the other actor enormously, although both knew not to take the crush seriously and Elisabeth was married), or a mixture of both. This was eventually Deconstructed several decades later in "School Reunion", where the Tenth Doctor meets up with Sarah Jane again and it's played like she's an Old Flame. The Doctor eventually admits he'd been in love with her, but knew he couldn't do anything about it because of their species difference, with the implication that she feels the exact same way.
Leela, ironically as the actors famously disliked each other. Leela was Ms. Fanservice and pitched as being sexier than Sarah - both were likely asked to play this up.
Romana II in the 1979-1980 era, due to real life bleeding into the production as Tom Baker and Lalla Ward were dating (and married a few weeks after she left the series). It's been noted that you can tell when Tom and Lalla had had a fight (and they even broke up briefly at one point) by how the Doctor and Romana interact on screen.
There's a very random and slightly distracting amount of sexual tension between the Doctor and Bettan in "Genesis of the Daleks".
Tension has been noted between the Fifth and Sixth Doctors and Peri, which is generally accomplished by Peri simply walking into the room.
The Tenth Doctor and Rose. Even in their last goodbye, it takes a clone of him to admit his feelings.
The Eleventh Doctor and River Song have ridiculous levels of UST. Which isn't surprising, considering the Timey-Wimey nature of their relationship, it's entirely possible that from River's perspective, they've, ah...resolved it in her past, (and therefore his future). Now quite firmly resolved, with only a mild stretch of the imagination regarding River's prison nights. Justified in the context of the show in that the two are married.
At least three cases of one-sided UST have been noted: Martha Jones wanted nothing better than to jump the Doctor's bones in "The Shakespeare Code" when they briefly shared a bed together (for sleeping only) but he'd have none of it. In "Flesh and Stone" Amy Pond - experiencing conflicting feelings over her impending marriage - attempts to resolve the UST by seducing the Doctor. And Osgood, whose crush on the Doctor - especially the Twelfth - is undisguised, goes so far as to aim a suggestive remark his way in "The Zygon Invasion". The Twelfth Doctor and Kate Stewart also exhibit UST when they meet in Series 8 and 9, but both seem oblivious to it.
Head Adelle Dewitt and her head of security Laurence Dominic ooze UST during their morally grey power walks. At least, until: he was revealed to be a spy. She sounds a lot like a jilted lover when chewing him out.
Topher and Dr. Saunders, at least until the arrival of Bennett.
Eureka: Carter and Allison. Everytime it seems to make progress the Reset Button gets mashed handily. Not as of the season 4 finale. Carter and Allison are still together (though more time travel was still involved to keep Allison alive)
Eureka and Warehouse 13: During the Crossover of these two shows, Fargo and Claudia had a full serving of geeky sexual tension in the first half of the crossover, which looked like it would go nowhere until Claudia's boyfriend dumped her at the end of the first half. In the 2nd half, back in Eureka, they got their Crossover Ship sailing, making out after they geekily disabled a mine that would've killed them.
ER: Between Dr. Mark Greene and Dr. Susan Lewis, Dr. John Carter and Dr. Anna Del Amico
Farscape: The UST Between John Crichton and Aeryn Sun in the beginning had its own gravitational field. Even after it was resolved, their unstable relationship made it feel like it never really got resolved (thus averting Shipping Bed Death) until the condensed, mini-series fifth season.
The Fast Show: Ted and Ralph. Virtually all of the humour in their sketches stems from the fact that Ralph has a crush on Ted, but is far too shy to confess his feelings, despite his constant, desperate, painful efforts to. Ted, meanwhile, is too embarrassed to admit he already knows, and, according to Word of God, too closeted to admit he feels the same way. Until the finalepisode, at least.
Firefly: The merchanteer Serenity had several couples with UST and played with the concepts with others.
Mal and Inara. He's a captain, she's a whore. He's a petty thief, she's a high class companion and way out of his league.
Simon and Kaylee. They both obviously like each other, but things tend to go badly when they attempt to do something about it. The U turns into an R in the Big Damn Movie.
The entire concept was skewered in the episode "War Stories". Wash declares his suspicion of UST between his wife Zoe and captain Mal. Mal is dumbfounded by the accusation: while they do have a long history together, their relationship is merely that of old friends and war buddies. Once the situation has resolved, Mal tries to make a show of it by insisting that he and Zoe kiss to "resolve the sexual tension". There's so little romantic chemistry between the two, (if you look at their eyes when they're acting like they're going to kiss, you'll see that they can't even look at each other and are frantically trying to look in any other direction out of embarrassment/unwillingness) that even Jayne, the crudest and more lecherous member of the crew, is unnerved by it.
Zoe: [Voice completely unemotional and with no desire at all] Take me, sir. Take me hard.
Jayne: Now somethin' about that is just downright unsettlin'.
There is also plenty of tension between, surprisingly, River and Jayne.
Niles and Daphne, in one of the most drawn-out examples, with some absolutely brutal Moment Killers and a plethora of false leads. Opinion is mixed on the effect of them getting together on the last seasons of the show. It ran for four more years thereafter, and even after they did get together, it took them almost an entire season to consummate. Of the four seasons, they were married for two.
Frasier and Roz suffer this a bit as well, but nothing ever really comes of it.
Friends: That show lived off UST, primarily that of Ross and Rachel. They would resolve it once, then something will go wrong, cue angry break up, the UST will slowly build up again over time, over and over again. When Chandler and Monica started up a sexual relationship that deepened into love, it was a relief.
Game of Thrones: By mid-Season 3, Jaime's relationship with Brienne approaches that of an old married couple, and when they eventually have a quite affectionate reunion on Season 6, Bronn wonders if they are fucking.
Jess/Rory during the first half of the show. Jess is Luke's nephew and Rory Lorelai's daughter, so clearly it runs in the family.
Glee: Played this between Will and Emma in the first season (though, ironically, once they'd gotten together they broke up because she was too mysophobic to have sex). In the second season, the major UST is between Kurt and Blaine. Kurt swears they're just friends, but try telling that to audiences after their duet of "Baby, It's Cold Outside". Also Sam and basically everyone (Puck, Santana, Brittany and Kitty share this as well, very fluid sexuality leads to a lot of sexual tension)
Kurt and Blaine was FINALLY resolved in "Original Songs." THEY DO, BITCHES.
Gossip Girl: Done all sorts of backwards with Chuck and Blair. They start out by having sex, and have a friends-with-benefits thing going for a few episodes. Then they end up in a drawn-out UST storyline, which includes playing Dangerous Liaisons type games, Chuck pretending to be Blair's boyfriend during a blackout, and generally wanting nothing more than to get together but neither willing to really be the initiator. Even lampshaded by Chuck in one episode, when he can't perform with other women thanks to his UST with Blair (the subsequent plan to use her as sexual Drano doesn't go all that well).
House: House and every other regular character at one point or another. Mainly with Wilson and Cuddy, although he has a lot of it with Cameron early in the series, and a fair case could be made for him having it with Chase. In Cuddy's case, it's resolved as they get into a relationship, but then have a bad breakup and Cuddy leaves the hospital (done partly because Lisa Edelstein chose to leave the show rather than accept the pay drop for the last season)
Chase and Cameron also have it in season 4. Resolved in season 5, but then they break up in season 6, leading to a return of it during their divorce, specifically in Lockdown, when they get trapped in an exam room together.
In Hunter, partner LAPD detectives Hunter and McCall are obviously attracted to each other, and even admit that themselves, but manage to keep their relationship on a professional, Just Friends level, while dating other people. At least, they manage most of the time — one episode reveals that they actually did sleep with each other once.
iCarly: Freddie with Carly, Played for Laughs throughout the first couple of seasons, before becoming more dramatic in the later seasons. And of course Freddie and Sam.
Inspector Lynley: The Inspector Lynley Mysteries had this between Lynley and Havers - to such a degree that, in the PBS introduction to the first series' last episodenote "Missing Joseph", specifically, host Diana Rigg explained quite emphatically that Lynley and Havers Couldn't Possibly Feel That Way About Each Other No Way No How. The sexual tension could have crushed your average linebacker, and it had only just started to heat up at that point! One wonders how she would have explained away certain scenes in "A Traitor to Memory"note that one scene at the end by the bonfire, where she puts her hand on his chest to stop him leaving and the tension is so thick you couldn't cut it even with an Absurdly Sharp Blade, or "In Divine Proportion"note the infamous Cry into Chest that is so far beyond platonic the line isn't even visible any more, or "One Guilty Deed"note when they dance around each other while she is in pyjamas and he is in a towel, and you wonder which one of them subconsciously wants to jump the other more, or "Word Of God"note the even-more-infamous-than-the-Cry into Chest scene in her apartment where they realise they are each other's reason to get up in the morning!... Suspiciously Specific Denial, indeed!
JAG: Essentially the series spanning relationship of Harm and Mac, as half of their spats in and outside the courtroom liked to play with this. The wake of failed relationships also seemed to keep their UST alive and well, as both Harm and Mac would have a new love interest by mid-season or the start of a new season. This lasted until the final episode where they decide to finally get married.
Jake20: The title character and his doctor in this short-lived show. While they came close at one point to "consummating their relationship", the show was cancelled before anything could come of it. Word of God is that, had they been allowed to finish off the season, they would have ended up living Happily Ever After.
Jonathan Creek: Present between Maddie and Jonathan on several episodes. Refreshing in that both actors were well outside the established romantic lead type. If anything, Jonathan and Carla had it worse in the fourth series. Not helped by the fact that they dated, broke up due to a silly misunderstanding, and the next time they saw each other Carla was married. Whoops.
The Late Late Show: Somewhat unusually for a talk show, the chemistry between host Craig Ferguson and very frequent guest Kristen Bell seems to have elements of UST, especially in the episode where Bell came on slightly loopy from drinking cold medication, and addressed (and lampooned) head-on during an edition taped in France where Ferguson and Bell actually seem to have "a moment".
Law & Order: Early seasons imply this between DA Jack McCoy and assistant DA Claire Kincaid. After Claire's death, it's revealed that this was actually Resolved. There are also hints of this with Mike and Connie in later seasons.
Law & Order: Criminal Intent: This show has devoted partners Robert Goren and Alexandra Eames. The UST is finally acknowledged in one episode by Goren's brother Frank, who irritably tells Bobby to "take Eames to a motel and get it out of your system." Since Alex is Bobby's Berserk Button, you can imagine the response. This one is unusual in that it has a fairly non-sexual feel to it - they act as if they're in love with each other, but there isn't a lot of heat between them.
Also Elliot and recurring character Dr. Rebecca Hendrix. Even Olivia mentions it.
Later seasons have Olivia and ADA Rafael Barba, what with the lingering looks, held gazes, habitual post-case drinks (or Barba asking her out for the aforesaid, at least), and one particular episode which has them hoping to be "squabbling at eighty-five".
Law & Order: UK: It's strongly implied that DS Matt Devlin and CP James Steel both have feelings for CP Alesha Philips. It actually becomes fairly obvious in each man's separate, but similar reactions to the video of her rape—James seems thoroughly sickened by it and can barely stand to watch it. Matt can't watch it at all, but is clearly just as sickened by what he can still hear.
Leverage: Parker and Hardison. There's been plenty of teasing, but while Hardison totally gets that there's something between them, Parker has No Social Skills due to her neglected background: she doesn't know how to have a relationship, so she's totally oblivious. Either that or she's teasing him.
She's starting to realize something is going on.
Parker: So the thing is, I think that maybe I might be having feelings, like weird feelings, for...pretzels.
Hardison: Well, they're right here, when you want them.
After defusing the bomb in The Big Bang Job
Parker: You know what I'm in the mood for? Pretzels.
Hardison and Parker are officially together as of "The Long Way Down Job".
So do Sophie and Nate. Sophie and Nate get a Bedmate Reveal at the end of "The San Lorenzo Job". They were both pretty drunk and Nate doesn't even remember. So they don't talk about it.
Eliot had some with his Psycho Rangers counterpart. When they first spotted each other they started imagining how their fight would go, which slowly devolved into a samurai fight daydream. The two of them were smiling every time they cut into these daydreams. At the end of the episode they are sitting in the bar showing each other scars, as she is fiddling with the handcuffs.
Bo and Dyson had this, though only for a few episodes. They slept together at the end of the second episode
Bo and Lauren. So much longing. And eye-sex.
Bo and Tamsin. Sort-of. Though is falls right on the line of UST and Belligerent Sexual Tension with all the snarkiness between them.
Kenzi and Hale. It remains unmentioned until the season 2 finale.
Vex: Oh, go ahead and merge naughty bits already. Seriously?! Am I the only one seeing this?
Also Kenzi and Dyson, especially in seasons 3 and 4.
Luther: Between John Luther and Alice Morgan, he's a big black cop with anger issues. She's a small redheaded sociopath who commits multiple murders. And yet they tend to get into each others personal space and look a heartbeat away from hate sex.
Has Wayne Rigsby and Grace Van Pelt. At least once per episode there is some mention that one of them is attracted to the other one, and they've actually kissed once while he was under hypnosis. This doesn't look to get resolved any time soon, either: workplace rules prohibit them from dating. Later it got resolved.. Rigsby and Van Pelt got found out and it led to their breakup. Rigsby still loves Van Pelt, she started dating a new guy. They get engaged, but then the new guy turns out to be a Mook for Red John, and is shot. So, back to square one.
Jane and Lisbon have some UST, and a fair amount of Ship Tease.
Merlin: Has this with pretty much all of its main characters at some point in time, although the most persistent/popular ones seem to be Arthur & Merlin and Arthur & Gwen. Arthur and Gwen's Official Couple status gets firmly resolved in the series 4 finale when they get married.
The Mighty Boosh: Howard Moon and Vince Noir have this in spades. Vince is Amiguously Bi and Howard has no luck with women, and there's an enormous amount of sexual tension between them. They finally did hook up temporarily in 'Party', but only in a Fake-Out Make-Out and because they were in a desperate situation, and the show has ended without the two getting together.
In response to fan questions, Barrett and Fielding have stated that they find the idea of Howard and Vince having any sexual relationship to be disturbing, as the two are such idiots that it just seems wrong. They have said the characters have a "kind of love" for each other, but that they don't analyze it.
Moonlighting: This was the whole premise of a romantic Dramedy series starring Bruce Willis and Cybil Sheppard as private detectives. The whole show jumped the shark in the fifth season when they finally consummated their relationship; the tension was simply gone and it become yet another sitcom. Lampshaded in the series finale, a great example of the show's frequent Breaking the Fourth Wall. The two detectives come back to the office to find it being dismantled by people working for ABC; an ABC network executive tells them that viewers had enjoyed watching them fall in love, but after they'd already fallen they lost interest. note In reality the downfall of Moonlighting had a lot of other causes as well. Mostly it was the result of tons of behind-the-scenes problems that plagued the show for the duration of its run, ranging from script and episode delays to a writer's strike that struck mid-season to the declining quality of scripts to (infamously combative) lead actors who simply didn't want to continue working on the show. Bruce Willis launched his film career with Die Hard between the third and fourth seasons and Cybil Sheppard, reportedly never pleased with the long working hours, wanted more time off to spend time with her growing family.
Murder in the First: Terry and Hilda get this for the first two seasons. Finally they get involved with the third, though it's only briefly because they're threatened with being split up by their lieutenant when he finds out.
Murdoch Mysteries: UST is an apt description of the relationship between Detective William Murdoch and their Station House's Pathologist Dr. Julia Ogden, particularly in the first several seasons. Each of them in their own ways contributes to the difficulties; William seems somewhat lacking in social skills (such as dancing) and has trouble telling Julia how he feels, while Julia is torn by her professional ambitions and ambivalence over motherhood in the face of his desire for a family. This is made worse by the Victorian/Edwardian setting and the necessary restraint needed by social standards of the time, as the UST was just as strong (if not stronger) whenever their on/off relationship was actually on. It was at its highest possible level during season five when Julia was married to another man but the UST was eventually resolved when her husband pulled an I Want My Beloved To Be Happy and told her to be with William. Julia and William rekindling their romance closes the season. In season 6, the tension is back as they are together but unwilling to commit adultery when Julia goes through the unavoidable scandal of her divorce, capped by the murder of her husband, for which Julia was initially convicted! Eventually in season 8, They Do, but boy howdy have they earned it.
NCIS: Despite both characters having relationships with other people over the course of the show, Tony and Ziva have kept up an ongoing unresolved sexual tension for seven seasons and counting. The series intentionally capitalizes on the UST with episodes like "Under Covers", in which they are obliged to pose as married assassins and fake having a lot of sex in the process, and "Boxed In", in which they spend most of the episode Locked in a Box.
Lampshaded by Gibbs in "Smoked". "You two done playing grabass?"
Lampshaded by a (married) Hooker with a Heart of Gold who instantly sees the UST between Tony and Ziva. She offers to help get rid of some of Tony's tension (he's still very hung up on his ex, the Arms Dealer's Beautiful Daughter) and comments on how such a hyper vigilant woman could be so blind (Ziva has no idea what she's referring to).
Even lampshaded in McGee's book Deep Six, where the characters based of Tony and Ziva are in a romantic relationship.
Cote de Pablo's departure during the eleventh season forced the writers to bring an end to the UST. Ziva and Tony share a Big Damn Kiss before she boards a plane that takes her back to Israel. Furthermore, Michael Weatherly's final episodes three years later reveal that the UST did get resolved, as he and Ziva had "a fond farewell" before she left NCIS - which resulted in the birth of their daughter Tali.
Abby and McGee are a case of Resolved Sexual Tension. They were involved for a few episodes in season one, but since their break-up have continued to flirt and get jealous of each other's love interests.
Gibbs and Director Shepard used to go out—and more—and there's still an odd sort of flirty banter going on between them. While they still seem to have feelings, they don't bother to pursue them. Possibly because she's his boss. Later enforced by her death.
Noah's Arc: This is Noah and Wade's driving dynamic early on before they get together and when they reconnect after their break up and getting other boyfriends.
Northern Exposure: Joel and Maggie. Resolved, unresolved, averted, inverted, subverted, lampshades, and every other durn thing under the sun.
The Office (US): Pam and Jim from the American version. A lot of critical speculation said that its resolution would destroy the show. Its continuing strength even after dealing with the UST is a testament to the writing team.
Primeval: Had multiple USTs going on at any point during the shows run. The most prominent being Jenny (or Claudia depending on which series you're watching) and Cutter...this is never resolved. No worries though, because Connor and Abby picked up right where they left off in series 3, just in time for the show to be canceled. Looks like the show may be Un-Canceled though — at least two new seasons have been confirmed, starting to air in 2011, so it looks like something might well come of Connor and Abby's UST after all. Especially when you consider the two of them are effectively trapped alone with only each other for company, as of the Series 3 finale.
Shawn and Juliet are definitely showing what looks like some UST. Granted, it's not the premise around which the show is built. There was a moment in season 2 where Shawn and Jules almost kissed but Jules wound up rebuking him and in season 3 Shawn was turned down for a date with the girl from the aquarium because she didn't want to get in the way. Shawn and Jules FINALLY give into their feelings for each other in season 5, and start dating.
Shawn himself has a high school potential sweetheart "The one who got away" named Abigail. They met again at their high school reunion which led to resolving a lot of their UST. But just when Abigail seemed interested in pursuing a new relationship, Juliet was in Shawn's line of sight and he clumsily but politely backs out of it. Several episodes later he decides to go for it with Abigail and just as he gets popcorn for her, Juliet tries to resolve their UST.
Quantico: Clay starts out completely hating Shelby for having an affair with his father AND breaking his brother's heart. As they work on the Task Force together, the chemistry builds to the point that Caleb and Felix both think something will go down between them even though Clay is engaged to another woman. Eventually, Clay makes a heartfelt confession and asks that Shelby make him hate her again so he doesn't pursue her and go against his moral compass.
Queer as Folk: Positively runs on the unresolvedness of Stuart and Vince's relationship. They are best friends from childhood, and Vince at least has been madly in love with Stuart for years, and as the series progresses, Stuart seems to reciprocate in his own way. They kiss, dance together, hold hands, and at one point even make it as far as the bedroom before stopping the situation from progressing any further. This hesitation is implied to actually stem from their intense feelings for each other, as they both seem to feel that actually having sex would reduce their relationship to the same level as Stuart's constant one-night-stands. As of the finale they appear to be in a contented quasi-romantic relationship, but whether they have actually done the deed is left ambiguous.
Remington Steele: Beautiful, successful Laura Holt and dashing, mysterious "Remington Steele", the show's two leads, had this in spades throughout the entire series, as they worked closely together on cases for her detective agency.
Salem: Mary Sibley still has feelings for John Alden, and is torn between her allegiance to the witches' cause and said feelings. Anne appears to have a budding crush on him as well.
Scandal: Abby clearly has this for Stephen in spades.
Selfie: Despite initially professing not to even like Eliza, Henry is slowly growing to have feelings for her, and almost makes a move on her when she thinks she's flirting with him (she was trying to teach him about flirting so he could have sex and mellow out). For her part, Eliza has some feelings for Henry, as she mentions its bittersweet seeing him go off with someone else.
Sherlock: Sherlock and John continuously throughout the series, as well as in A Scandal In Belgravia between Irene Adler and the eponymous Sherlock, though it doesn't go anywhere.
Smallville: At some point on the show, everybody crushes after everybody. But almost never in pairs. Since half of the early crushes involve LanaLang and a one-episode-appearance meteor freak, Clark longing for Lana, and Chloe lusting after Clark to the point where she gets enraged and joins forces with Lionel upon seeing Clark and Lana kiss, the effectiveness of the UST depends very much on viewers liking that character (Though the Lana-stalker plots eventually fell away, Clana broke up and was buried, and Chloe eventually did mature out of her teenaged crush).
Used effectively in Season 4 between Carmela and Furio. It ends painfully when Furio panics about his feelings for Carmela, due to the fact that she's The Boss's wife, and moves back to Italy. In the season finale, it becomes an important factor in Tony and Carmela's separation.
And of course, Tony and Dr. Melfi, who knows him emotionally perhaps better than any other woman in the series—being his therapist and all.
Spaced: Daisy and Tim. Incredibly frustrating as the show ended because of Simon Pegg getting distracted with a movie career that came out of nowhere, just as the two seemed to be realising what the audience was rooting for since episode 1.
The sexually unresolved yet happy ending is heartwarming. The shot of the pair watching TV to the Lemon Jelly soundtrack feels very fuzzy.
Sports Night: Has it in spades between Dana and Casey. Then later between Dana and Sam.
One wonders if William H. Macy (the Real Life husband of Felicity Huffman, aka Dana) was cast as Sam because of the planned direction things would go between the characters, or if Sorkin played up that angle in response to the casting. Chicken and egg.
Stargate Atlantis: Sheppard and Weir provide the UST. Until Weir leaves the show in the fourth season premier, that is.
O'Neill and Carter are the epitome of this. Behind-the-scenes forces make any romantic feelings between them genuinely appear and disappear at apparent random (as opposed to Will They or Won't They?). Nonetheless, it showed up least once a season. It's so obvious it hurts. Them being married in at least one AU doesn't help. And kissing. Word of God votes in favor of the romance. The producer stated in an interview that while it was never presented officially on the show, "in my mind, they got together [after O'Neill retired] and have been together ever since." A deleted scene from one episode strongly suggests that O'Neill and Carter finally got their resolution on.
After O'Neill left the show, Daniel and Vala took over the job of teasing the audience about the potential of their relationship, though it was less heavy-handed in that case. Well, at least until they got together in a timeline that ended up being reset.
Daniel and Janet had it as well. This was notable because the writers themselves didn't plan it. Daniel and Janet's actors made a conscious decision to throw it into their scenes together quite deliberately to give the fandom something to talk about, and the show's director let them do it. It did get the fandom talking about it.
Star Trek: The Next Generation: Captain Picard and Doctor Crusher have UST from the very first two episodes (wherein he nearly has a stroke upon first seeing her and then again when she comes on to him while under the influence) to the last (wherein they eventually (in what may be an averted alternate future) marry, and then divorce, though she keeps his name). This gets particularly bad when they are telepathically joined in season 5, discover and openly discuss the UST, and still choose not to resolve it.
Well before any of its spin-offs played with the idea, this show experimented with the idea with the relationship between Captain Kirk and Yeoman Janice Rand: both had a mutual attraction to one another, but Kirk's position as Captain, and his feelings of responsibility as her commanding officer, were explicitly stated as the reasons why they didn't. The episode "Miri" is as close as it got to being resolved, as Rand declares her love for him, but Kirk just can't quite do the same with her... and she's gone after the next episode.
More than a few fans perceive Kirk's friendships with both Spock and Dr. McCoy as brimming with UST.
B'Elanna and Tom Paris, for the first few seasons at least, before they get married.
Then there's Seven and the Doctor. He started to become attracted to her in "Someone To Watch Over Me" and certain episodes hinted that she may have had feelings for him as well.
Sue Thomas: F.B.Eye: Sue Thomas and Jack Hudson. A few episodes plays with this, e.g. "The Newlywed Game" where the pair goes undercover as a married couple, and "The Kiss" where Jack fakes having a affair with Sue to maintain their cover at a law office they infiltrated.
Terminator: The Sarah Connor Chronicles: Has enough of this between John & Cameron to lift a blimp. Semi-resolved during a scene in the final episode where John checks her power source, inside her body. It is played as losing their virginities, complete with nervousness, awkwardness, instructions ("take your hand and put it here") and reassurances ("that's good, John") and that it doesn't hurt. This charged scene is open to interpretation. It is possible Cameron is deliberately resolving the sexual tension, especially when she asks John how it feels in there and he has to reply. "Cold." Yet there's no kiss or anything...
Cameron is really one of those characters who has sexual tension with everyone. There's tension between her and every single one of the show's lead characters, belligerent or otherwise. There's even some between her and Riley, and she's flat-out terrified of Cameron.
Torchwood: Jack and Gwen. Whoever else Jack might be shagging, it's those two who've gotten the longing gazes and a certain nearly incendiary shooting lesson. It still exists by Children of Earth, even though Jack and Gwen are with different people. In commentary, John Barrowman did refer to this in reference to the scene where Jack learns of Gwen's pregnancy before Rhys does and when Rhys learns of Torchwood. Their UST is highlighted again in Miracle Day, though this time in a much darker context.
Damon and Elena. The first 3 seasons were nothing but painfully frustrating D/E sexual tension until it was finally resolved in season 4. After their break-up in S5 and break-up in S6, again Delena UST leaked out of the screen until they got back together.
Klaus and Caroline were another frustrating UST eyesore, only worse because it was longer and constantly covered up by Caroline until it was eventually resolved by a passionate one-night stand in the woods.
Stefan and Katherine (Season 2/Season 5), Damon and Katherine (Season 2).
Stefan and Elena post-break up in S4 and S5, but it slowly lessens and fades away altogether by S6 when they both move on.
Damon and Bonnie. They had a few tense scenes in Seasons 1 and 2, though they both openly hated each other. It gradually faded as the series went on until Season 6, where the first few episodes were nothing but UST-filled Bamon bickering. In Season 7, the UST gets charged UpToEleven. Sometimes you even catch them staring at each other's lips.
True Blood: This show is laden with sex and sexual tension. Sookie and Bill have tension from the first episode until their initial get together early in the season, but the series most prominent UST is between Sookie and Eric from the moment they meet until their UST's resolution in season 4. Sookie and Alcide have a fair amount as well. Jason and Jessica get hot and bothered in seasons 4 and 5. Tara and Pam start and resolve their UST in season 5.
Victorious: Jade and Tori have a ridiculous amount, be it of the most extreme Foe Yay-meets-Les Yay variety, but harder to deny as the series wore on.
Beck and Tori early on, but seriously downplayed by the end of the first season onward. By Tori Goes Platinum, however...
Cat and Robbie. Robbie and Beck also seem to have more than a bit as well, possibly Lampshaded in A Breakfast Bunch.
The Walking Dead all but ignored UST for it first three and half season's except for a few teases. However the back half of season 4 included plenty of tension between Daryl/Beth and Rick/Michonne and a whole new relationship forming between Bob and Sasha.
Aggressively averted with Pete and Myka. When Myka was under the effects of an artifact that forced her to act on her suppressed desires, you'd expect this would lead to her kissing Pete. She punched him instead.
Winked at with an earlier Warehouse pairing, a lot like our two agents, who actually did fall in love.
In "Duped," Pete finally discovers that the Myka he's with is a fake when she kisses him.
Pete: The real Myka would never kiss me – not if her life depended on it!
There is a little here and there, though. For example, she admits she named the ferret Pete because "it's annoying but cute."
Sexual tension may be averted with Myka and Pete but it's played for all its worth between Myka and HG.
HG: How do you say goodbye to the one person who knows you better than anyone else?
Had Josh Lyman and Donna Moss. They eventually did resolve it, but they dragged it out over six and a half seasons, which made the resolution feel weird, like you expected it for so long that you stopped expecting it. A well-done case of Will They or Won't They?. Josh and Donna's relationship is alluded to and quasi-lampshaded repeatedly. A few of dozens of examples:
In "Seventeen People" (season 2, episode 18), Josh says "If you were in the hospital, I wouldn't stop for a beer," and Donna replies, "If you were in the hospital, I wouldn't stop for red lights." And that's after Donna compared Josh to her ex-boyfriend.
In "The Women of Qumar," (3x8) Amy asks Josh, "Are you dating your assistant?"
In "Commencement" (4x22), Amy says to Donna, "Are you in love with Josh?" And Donna never answers, of course. It's a pristine example of lampshading, and a metaphor for UST: the question is asked and never answered, just like the UST appears but is never resolved (or rather, not for a long while).
In several episodes, their relationship is acknowledged by other cast members. They talk to Josh about him being jealous over her, and Joey Lucas addresses it in one episode. In "Gaza" (5x21), after Donna is in an accident, Leo asks Josh if he needs to go see her, and of course he does.
Sam Seaborn and about half a dozen women over the course of the series. The writers would introduce them, let Sam have four or five episodes of UST with them, and then make them disappear without any explanation whatsoever. They did this with Mallory, Ainsley Hayes, Connie Tate...
Who's the Boss?: Tony and Angela — to the point where their Dance of Denial annoyed even the secondary characters.
Wire in the Blood: Tony and Carol have bucketloads of it. They share a ridiculous number of charged moments, complete with looks of longing, almost-kisses, a fair amount of jealousy and plenty of subtext.
Wizards of Waverly Place: Many fans commented on a particular, serious case with Real Life actors Selena Gomez and David Henrie, who play Alex Russo and Justin Russo. The case is serious due to the actors' strong chemistry that turned the film's heartwarming, brother-sister scenes into almost romantic, flirtatious moments. And because it's a Disney production.
Xena: Warrior Princess: Xena and Gabrielle. So much. And Xena and Ares. And Xena and Hercules (Although given that they had sex in the backdoor pilot, maybe not so unresolved?). And Xena and Iolaus. And Xena and Draco. And Xena and Borias. And Xena and Lao Ma. You get the pattern? Then Gabrielle and Joxer.
The X-Files: Mulder and Scully. According to internet lore, the term and abbreviation for this trope originated in the online forums when the show first aired to describe their relationship, though the writers didn't intentionally start capitalizing on it until nearly half-way through the show's run; Word of God maintained that there was nothing romantic between Mulder and Scully. However, the chemistry between the leads looked pretty non-platonic from day one, and whether or not it was intentional, nearly every episode from seasons 1-7 had at least one moment highlighting this and driving fans crazy. It finally was resolved in season 7 (offscreen, to the dismay of fans), and had arguably started drifting into Romantic Plot Tumor territory. Perhaps ironically, both the episode where the UST is eventually resolved and "The Unnatural," which contains probably the most shining example of their UST in a Hands-On Approach baseball lesson, were written and directed by the actors themselves.