Oooh, someone pinned this thread, so now I feel like I can resurrect it after more than a year!

I highly recommend anything written by
Melissa (
Snapelover). She's done a total of seven fanfics to date, all of which are linked in her signature in her user profile. My two favorites, though, are her first fic,
The Love of a Wizard, and her nearly-complete WIP,
Dream a Little Dream.
The Love of a Wizard is probably the greatest fanfic I've ever read, and it really ought to be up there with the "classics" that most people involved in the HP Internet fandom have read or at least recognize. It's probably the first story involving an OC that I've ever enjoyed, and the way Melissa interweaves the parallel world of Claudia, Emma, Sirius, Lupin, and Snape with the canon is amazing.
Dream a Little Dream, however, is a serious rival for the immortal TLoaW. I can say that it's a Hermione/Snape story without giving serious spoilers as the ship is part of the thread title, and even if you don't normally like teacher/student romances, I promise this one's different. It made me a total SS/HG convert, and I've rarely seen such wonderful characterization.