Spike/Xander/Anya
Power Center by Saturn Girl. NC-17 overall.
S6, AU-ish. No, no, don't read this story first. Read the whole snarky, achey, sexy, brilliantly funny Power Struggles series, which is S/X right up to the end -- when Anya solves things with grace, intelligence and aplomb. Not to mention damn good taste.
Power Play | Power Struggle | Passion's Prisoner | Power Shift | Power Center
You've Got Spike by Michelle/Shelly. NC-17. WIP.
Near future. Yes, it's a work in progress, but... But, damn, it's worth reading. Hilarious (as is all of Michelle's fic) and hot and silly. Spike has moved in with Xander and Anya. Just because, you know, he needs a place to stay. And a phone to use. For his phone-sex job. Yeah. Like that. If you were Xander and Anya, and had to listen to him say those things to strangers, who don't deserve him and don't pay nearly enough for his company, wouldn't you want a Spike of your very own?
It's All My Fault by Nancy. R/NC-17.
(She rates it R - I'd say NC-17, but only because there's enough description for me to go 'guh'. YMMV.)
A sadly sweet Anya POV. Giving her lover what he needs most in the world, and slowly losing him because of it.
Absurd by Mcee. PG.
Xander and Anya take care of Spike after his torture at Glory's hands. It's a wee bit AU, but gentle and painful and heart-tugging.
Let's Talk About Sex by Estepheia. NC-17
A long, humorous, and carefully-developed relationship story, set in a less-depressing BtVS season 6. 
So Happy Together by Ladycat. NC-17
Threesome sex in the bathroom at the Bronze. Which would be Very Very Wrong. So obviously it didn't happen. 
Spike/Xander/Buffy
Lollipop by Ladycat. NC-17
PWP, probably season 6 AU or later, but it's not episode-specific enough to tell, or matter. What it is, is exceptionally hot, and surprisingly sweet.
Vacancy by Ladycat. NC-17
This, on the other hand, is dark, and nasty, and Wrong. In the Wrong way of Wrong. And most likely set between Wrecked and As You Were. Spike uses Xander to use Buffy, because Buffy pretty much wants to be used. Nobody's happy.
Lest I be accused of...something. Misogyny? Not playing fair? I'd *love* to have more good S/X/female stories to rec. There just aren't many out there.
I have the same sort of Willow-Sue issues with the few S/X/W stories I've seen, as I do with plain old Spike/Willow stories -- she's not usually in character. Most of the characters aren't usually in character.
Spike/Xander/Drusilla... I'd love to see, done well. With a one "kitten" per story limit. I'm still waiting. (I considered offering Scorpio's "Kitten" as an exception to that, because at least it plays with the stereotype by offering a Xander who *is* catlike, when vamped - but it's unfinished, plus I'm queasy about non-Wishverse vamped Xander. If he came that way, fine, but I don't like watching our Xander become evil.)
Spike/Xander/Buffy... I've found a few (pointing upwards...) that work for me, but they really have to work for me, to make me buy, or want to read them.
What doesn't work for me in a threesome story?
One of my biggest problems with any threesome story (aside from the odd one that's specifically about someone getting left out, like Nancy's S/X/Anya above, or Ladycat's Vacancy that's twisted and wrong on purpose) is that I really can't deal with stories where one of the partners is a third wheel. Where it's obvious that the author thinks the Big True Love is between X and Y, so Z is just there for the kink factor, or to show via his reflections on the subject, how wonderful the X/Y relationship is. I will run screaming from such stories. If the characters are aware of it, and it's not out of character for the third wheel person to put up with it, if there's a good reason, if it's explored, that's another story. But for instance a Spike/Xander/Buffy that's really about the Glory of Spuffy that's so shining and strong it can handle anything, even the introduction of another sex partner? Really not my thing.
Though I'll admit that I'm probably more likely to put up with it if the Couple Of Troo Luv is S/X, and the third wheel is another character. ;-) I'm not a big fan of it, but I'm not zen enough to totally rise above my One True Pairing prejudices.
Suggestions? Know of one I haven't read? E-mail me, by all means.