Having said all that, I know I'm going to leave out at least one really brilliant fic and kick myself over it later, so this list is prone to updates without notice. Here we go, again in no particular order:
Anything and everything by Erica H. Smith. Denizen of the Deep is a thoroughly delightful fairy tale centring around one of my favorite minor HP characters. Paraphrase: Sublunary is an addendum to Chapter 6 of my own fic If We Survive and a superb character study of Snape. Postscript: Like Gold, which takes place a few months after the end of If We Survive, made me cry with happiness the first three times I read it. And then there's Erica's wonderful trilogy crossing the HP universe with the works of SF author Lois McMaster Bujold, which even non-Bujoldians can understand, but which fans of LMB will most enjoy and appreciate: Marks and Scars, Without Enchantment and No Great Magic. Erica's work is lyrical, witty, and at times deeply moving. And I'd better shut up about it or I won't have time to list the rest of my favorites before bedtime.
Harry Potter and the Heir of Slytherin by DrummerGirl. DrummerGirl writes the most authentically JKR-ish style I've ever seen, and this novel-length fic really has the feel of a genuine Year 5 story -- and yet it's full of fascinating original ideas and introduces us to at least one truly likeable and plausible new character. If it runs afoul of the occasional fanfic cliche, at least it's honestly done -- the author hadn't read any other HP fanfics when she wrote the outline for hers.
Imperius Quidditch by Alec Dossetor and Teri Krenek. A superb character study of the young Tom Riddle, and of the challenges and temptations of power. Plus, the whole idea of Imperius Quidditch is the most brilliantly original concept I've seen in a long, long time. Don't be misled by the leisurely, rather old-fashioned tone of the opening paragraphs: this is a fast-paced, gripping story.
Down from the Tree: Actions by Melissa A. A marvellous story, particularly when read in tandem with Arabella's sequel Down from the Tree: Consequences: it provides a very plausible scenario for what happened on the night Sirius pulled the infamous stunt that almost got Snape killed. Smooth storytelling, wonderful characterization, and a very balanced portrayal of a situation that has previously tended to polarize fans.
The More Is My Unrest by Jedi Boadicea and Arabella. This is the one and only truly plausible, canon-consistent Draco/Ginny fic I have ever read. Although it isn't really D/G. Just go read it, you'll see what I mean. The characterization of Draco is wonderful -- not in the least glamorized or white-washed, and yet at the same time it's hard not to feel for him.
Acts of Necessity by Jedi Boadicea. This is the funniest humorous fic I've ever read, and contains absolutely brilliant, bang-on portrayals of both Severus Snape and Gilderoy Lockhart, in a socially awkward situation that results in Lockhart getting a delicious and well-deserved comeuppance. I found it both hilarious and enormously satisfying.
And now I have to go to bed, so any more recommendations will have to wait for another time!