Buffy and Faith have a ridiculous amount of Les Yay, which translated into a ridiculous amount of Foe Yay after the latter's Face-Heel Turn. This◊ (very small) GIF sums it up, including Buffy's recurring dreams about Faith in which they 'make a bed' together.
Despite Faith continually having trouble mingling with the rest of the group she loves to get Buffy to go slaying with her, and she makes no secret that slaying gets her horny. Talk about strange foreplay.
Their 'outing' in "Bad Girls" was pretty much a date, starting with Faith interrupting Buffy's class with a "hey girlfriend" and drawing a heart on the window, continuing to a bout of slaying, and ending with some very intense dancing.
The commentary of that episode told us that that scene was based on an experience the writer had in school, where he witnessed a girl climb up to the window and do exactly that (minus the "staking") sign, and another girl promptly climbed out of the window. One assumes that they didn't go out to slay vampires.
Faith is blatantly jealous of Angel, paralleling Xander's hatred towards him, and in general is both endlessly curious about Buffy's love life and insistent on convincing her that men are worthless and can't be trusted. Indeed Faith is almost desperate to get Buffy to agree with her on how she views the world, as if she's looking for validation.
Their Foe Yay is almost a direct mirror of of that between Buffy and Angelus. Faith's attempts to bring Buffy over to the dark side compared to Angelus' manipulations, and the Buffy/Faith fight in "Graduation Day" being just as tense and emotionally charged as the Buffy/Angelus fight in "Becoming, part 2". Buffy is also willing to be enormously forgiving and understanding of Faith after she kills a man, lies about it and tried to kill Xander, just like her forgiveness of Angel after he regains his soul.
The infamous Take That Kiss Faith gives to Buffy, which was originally scripted to be on the lips instead of the forehead, and which Buffy returns when Faith's in her coma.
What starts out as Faith giving a Breaking Speech to Joyce fairly quickly ends up as Faith ranting about Buffy not liking her anymore.
Faith: "[Buffy] was over us a long time ago. Too busy climbing onto her new boytoy to give a single thought to the people that matter..."
What makes this better is that several times attempts were made to portray Faith as straight, from being disgusted at a drug addict hitting on her to saying she won't go downtown on another Slayer. She seemingly makes an exception for Buffy.
Angel and Giles get really close on a peculiarly frequent basis given that they rarely interact, while Giles was shockingly quick to forgive Angel for killing Jenny.
The final between Drusilla and Kendra has been described as "almost sexualized" and "a G-rated lesbian interlude". This foreshadows the Les Yay between Drusilla and Darla in Angel.
Xander has made innumerable homoerotic comments and been in almost as many unintentionally romantic/sexual situations with other men. Coupled with his frequent frantic denials and mild homophobia, this makes it very easy to read him as a closeted bisexual. Joss Whedon has gone on record saying that, in the early seasons, he knew he wanted either Xander or Willow to experiment with their sexuality but didn't yet know which one, so both of them got a lot of... moments.
Xander explained why Willow would find Oz attractive. Later, he explained why Buffy would find Spike attractive, which lead to the immortal line "I am not having sex with Spike - but I'm starting to think that you might be."
The first time he sees the Initiative's Elaborate Underground Base: "Can I sleep with Riley too?" He sounds at least as sincere as he did when he half-jokingly suggested a "pre-birthday spanking" for Buffy in season two.
That season seven episode where the intertwined sexual history of Buffy, Spike, Xander and Anya is explained to a girl Xander likes. In response, she asks if any of them haven't slept together, and Xander and Spike share an awkward look.
Spike calling Xander a 'nummy treat'.
The scene in which Xander confronts a Jerk Jock about him being a werewolf, only to turn out that he's just gay. And not only did Xander give him the courage to come out of the closet, but now he thinks Xander's gay as well.
It's blatantly obvious that Giles and Ethan were involved in one way or another pre-series. Writer Jane Espenson has said that she thought the two of them had a past involvement romantically.
It certainly helps that nearly their every encounter comes across not as "Why did you turn away from the dark side, Ripper?" so much as "What happened to us, Ripper?" Either that, or it's a noticeably more serious case of Go Karting with Bowser considering how often Ethan's visits get people killed.
Ethan: Oh, we go back - we go way back.
Parodied in "A New Man" when Giles thinks Ethan is drunkenly hitting on him when he's really hitting on the waitress.
Also in "A New Man", Ethan describes them as "a couple of old mystics" in about the same tone of voice he'd use to call them "a couple of old queens" (maybe more of a British thing), and after Ethan tells him that "the night is still [their] time, the time of magic", the scene immediately cuts to Willow and Tara sharing one of their most obvious 'MAGIC IS HOMOSEXUALITY' moments.
The comics bring us Satsu, a Japanese Slayer who wakes Buffy with a True Love's Kiss. Buffy is touched but tries to invoke It's Not You, It's My Enemies to break up with her. Then they sleep together, twice, the first time Buffy saying it was one of the best nights of her life. Satsu, for her part, is upset when Angel returns because she sees him as a threat to her love interest.
Buffy and Willow have a very developed relationship that strongly rivals the more prominent Buffy/Faith Les Yay. This is more noticeable in Seasons 1, 2, and 3 despite their romantic interests. It still appears from time to time in the final three seasons, specifically in Season 7's "Same Time, Same Place". Even ignoring the constant physical touches, the incredibly close emotional and supportive bond they have for each other is exceptional even amongst the other True Companions, including constantly sharing looks at the same time and mimicking each other's actions (e.g. "Teacher's Pet").
The interactions between Buffy and Willow in "Welcome to the Hellmouth" are very easy to see as "love at first sight", particularly note the longing looks when Cordelia runs Willow off at the water fountain and the decisive way in which Buffy chooses Willow, despite Cordelia and Willow separately telling her that she can't have both and that choosing Willow will instantly mark her as unpopular.
Buffy hug-tackling Willow (out of pure glee) when she learns Willow is staying in Sunnydale to attend UC Sunnydale (the same college Buffy would be attending) in Season 3, and they get coffee together afterwards. This all despite Willow getting woo'd by Oxford, Harvard, Yale, and any prestigious academic school one can think of.
Willow plying Satsu for details after she found out she slept with Buffy in Season 8. Willow also goes so far as to ask if Buffy made a certain noise during the sex.
Kennedy finds out about Buffy and Satsu and is threatened enough about how close Buffy and Willow are she makes death threats. Everyone Can See It indeed.
Another, much more obvious (nearly blatant) example occurs later when Buffy and Willow are about to rescue Satsu. Note that Willow brings this up entirely out of the blue, and she's the canon lesbian.
Willow: I'm sayin' it's a good thing you didn't try your little experiment on me. 'Cause it wouldn't have worked, Summers. You're not even on my list.
Buffy: What list? There's a list?
Willow: Yep. And you're not on it. Not my type.
Buffy: Oh, yeah? Well, then, you're not on my list either!
Willow: Good. I don't want to be.
Buffy: Sure you do. It's a very distinguished list. It has people like um... Judi Dench and Eleanor Roosevelt]] on it.
Willow: Eww. Just got the visual.
Buffy: And other people too. Those are just examples off the top of my head.
At the start of season nine, Buffy wakes up from a wild night with Willow, the indication being they slept together. Willow doesn't deny this happening. Throughout the series Willow has always had two love interests: the current boy/girl she liked, and Buffy.
Willow: I know you do. [Buffy strokes Dawn's hair]
The Buffybot talks about Willow "servicing" her in Season Six. When you add in the pre-existing Les Yay with Buffy and Willow, Willow's overt homosexuality, and the Buffybot's original purpose...the start of season nine pretty much confirms Willow did it with the bot and currently it's looking very much like she's interested in the real thing.
Buffy and Kendra in season 2, kind of a "warm-up" to the Buffy-Faith relationship next season.
Buffy and Cordelia ("I know you share this feeling we have for each other, deep down...")
It's heavily implied that there was something, even if we're not sure exactly what, between Warren and Andrew during season 6. Andrew was pretty obviously in love with him.
Andrew:(Crying) He left me. He flew away and left me. How could he do this to me? He promised we'd be together, but he was just using me. He never really loved... hanging out with us.
Glory induced this way too much, to almost Depraved Bisexual levels. Particularly with Dawn: changing in front of the girl, playing with her hair, accusing her of "trying to get a peek at [my] unmentionables?" Just the start.
Dawn/Glory goes both ways:
Dawn: All of a sudden Glory's standing right there in front of us, all skanky and blonde and thinkin' she's all that just 'cause some bumpy heads kiss her stinky feet ... (pauses, very quietly) She does have nice feet.
Glory talks about having "girl time" with and says to her "Let's have big girl fun".
She also talks about having "girl time" with Tara and licks her hand in a sensual way.
Glory seems genuinely hurt when learning that Buffy broke off a date with her male alter-ego: "She turned us down?"
The episode "Dead Things" had Buffy break down crying in Tara's lap over Coming Back Wrong, begging Tara to not forgive her.
Tara, of all people, is Buffy's dream guide in "Restless".
Xander is very complimentary of Dracula after being hypnotised by him in the season 5 premiere, and in the comics Dracula is blatantly affectionate towards Xander. At least one writer has said he believes Dracula has romantic feelings for Xander.
In the season 1 finale "Prophecy Girl", while walking in the sewers together, Xander yells at Angel for staring at his neck.
Andrew and Jonathan in "Storyteller". You might half-expect them to snuggle in the bed together.
Spike's victims in Season Seven were mostly women, but there was Holden Webster. Who Spike told his name to before siring him. Is the implication that he picked him up too? (Although Spike was under the First Evil's control at the time, it arguably could be considered More Than Mind Control.)
Spike quite obviously adores Angelus almost as much as Drusilla, and is ecstatic when he briefly returns in season 2 (at least until he starts seducing Dru). He loathes Angel, but less in a "betrayed the cause" kind of way and more of a "jilted boyfriend" way.
In "Once More With Feeling" the demon Sweet plans to take Dawn (who he incorrectly assumes summoned him) back to the underworld to make her his queen. When Xander is revealed as the once who actually summoned Sweet, he asks if that means that he has to become Sweet's 'queen'. Sweet eyes him and says, "It's tempting".