As I just finished explaining to lizbee, it's a clever, well-written, HBP-compliant Snape genfic -- WiP, but the author seems to be updating pretty regularly. I have a few niggles, but that's inevitable. The best thing about the fic is how dryly funny it is, and how the author manages to portray Snape's interpretation of certain events and people as being eminently logical -- even brilliant -- yet at the same time completely wrong. She also shows a plausible flip side to many of the events we're familiar with from canon, as well, which gave the story a freshness that it might otherwise have lacked.
Anyway, enough of my promotional blather: The Diary of the Half-Blood Prince, Part I by CareCrystal.
ETA: I originally said that this fic had some bits in it that were movie rather than book canon, but I've since realized that said bits were in the books and that I am an idiot, so I withdraw the objection. Also, the latest chapter explains everything I thought was slightly "off" in the fic and the rationale is sheer brilliance, so all I can say is -- GO READ THE FIC.