And now, one long...
Dumbledore's Army and the Year of Darkness by Thanfiction
(can be found on its own website here or on fanfiction.net here)
approx. 250,000 words, rated R or M, warnings for extremely graphic violence
The story of what happened at Hogwarts during Deathly Hallows from the viewpoint of Neville Longbottom, who becomes the Commander of the DA. It quickly becomes apparent that the summary Neville gave Harry at the end of the book about what they'd been up was an understatement, but then isn't that just Neville all over?
There is so much I loved about this, from the fully-realised characters that are merely peripheral in canon, to lots of wonderful little world building details, witty dialogue and hilarious moments, and the final battle, which is more heart wrenching than ever when you are fully invested in every single person.
Quote: "I mean," Neville shrugged, "sure, we all know Dumbledore had something planned with Harry, but we don't know what, and we're not waiting to find out. If Harry can stop You-Know-Who first, that's brilliant. But these are our freedoms too, and we're not sitting back and waiting on someone else's plan." - Neville to Aberforth
...and two short...
The Two-Step of Death by lady_game</lj>
10 x 100 words, PG13
Ten drabbles focusing on ten characters post Halfblood Prince, beautiful written, with so much said in so few words.
Quote: He likes his magical eye, because it means he can look over his shoulder without even having to turn his head.
That Other Fellow by sciathan_file</lj>
guess-timate of 3,000 words, PG13 to be on the safeside
A wonderfully clever story showing the war through the eyes of the Muggle Prime Minister. Remus and Charlie feature, but Kingsley, or rather the PM's view of Kingsley, stole the show for me.
Quote: "Make a call to this department," replied the Prime Minister at a length, handing his assistant a slip of paper. "Tell them that it is a matter of extreme government secrecy and diplomatic delicacy and, if you have any issues, I shall be here running my half of the country. If any wand waving madmen happen to make it through the fireplace, I'll scream."