Much to my surprise, I've been following a couple of WIPs and thought I'd pass on the joy...
Scale of Dragon, Tooth of Wolf by
ebonlock is a fascinating murder mystery in Holmesian style. Snape is brilliant, short-tempered, and thoroughly bored by his incarceration. RL/SS
The Sensible Garden by
joanwilder is a richly detailed, thoroughly mature story. I can't summarize it, but it features two of the best OCs I've found in fanfiction, and deals with death, responsibility, grief, and love in full, living, color. HP/SS
And a third, lighter, shorter fic:
The Opportunist by
windypoint is a laugh in a few paragraphs. Don't drink while reading. The author gives this summary: Severus Snape has always had a flexible viewpoint on the appropriate use of Potions. SS/WG-P