"Seeker to Seeker" by BaronNomaw.
First read: 2004.
Second read: 2005.
Third read: 2006.
Fourth read: 2007.
EDITED: 2009: I have finally grown up enough or gotten enough distance to realise the writing in this is crap. The story I read back then was awesome and still is, in my memory, but I can't re-read this any more without seeing all the amateurish writing mistakes. Ignorance is bliss, indeed.
Notes: 61.000 words. I have read this so many times that if it were an actual book my copy would have fallen appart, I'm not actually all that sure it's been only four times, just let me confess to remembering it all during that awful trip to Disney Paris to keep myself entertained. It's a post-GoF fic, Lucius is still free and the boys' enmity is simply a schoolyard thing. One that can be overcome with Quidditch and risky bets. These bets are sex-related only because they want to humilliate each other, clearly! But dreaming Quidditch together and fucking like rabbits it's hard not to become friends. The war is not so far away that they can completely ignore it forever, though, so in later chapters things get serious. I find this fic hillarious, well-characterized and lovely all around. There's a scene-long sequel,
Draco's Homecoming, dealing with one of the events mention in StS in more detail from Draco's pov. And a sequel that's been a WIP forever and that didn't seem to keep up the quality of the original (although I did feel StS needed a sequel)
Power Play.