Fic: Servants with Torches (http://www.ravenswing.com/~mirrorgirl/servants.html)
Author: Jane St Clair
Rating: NC-17, I guess, though it isn't very explicit
Pairing: Romeo/Mercutio
Category: Very romantic, angsty slash

What better fandom for Valentine's? But R & J is surprisingly slashable and I've always thought this pairing has more potential in it than the canon one. Very atmospheric and brooding.