In Between Days by atrata. Snape/Harry, NC-17 (very not work safe).
You'll have to forgive me for this less than coherent rec, but I should have been in bed and asleep more than an hour and a half ago -- but there was no way I could stop reading!
The story is so yummily dark, and Snape is so... Slytherin. You can't tell what exactly he's up to until the very end. (A very outstanding end, in fact!)
I don't read fanfic for the sex scenes (I avoid PWP -- non-fanfic people: PWP = Plot? What Plot? Fics with little or no plot, just sex), but man, the sex in this fic was [tired brain tries to come up with something clever ... H-O-T-T? Hot-with-a-capital-H? zzzzz].
It looks like atrata finished writing this a few days ago, so I may be behind the times in reccing it, but if you're like me and missed it thus far do yourself a favor and go check it out!
And now: Bed. And hopefully yummy Snarry dreams. :)