Women Have Lovely Bones (PG) by tigriswolf. This is a really well-executed outsider POV, where Lilith schemes for the future and SPN canon is replayed through her eyes to an entirely different effect.
You Would Cry Too (If It Happened To You) (PG-13, early S2) by superbadgirl. I read this cracktastic story months ago, and it definitely stays with you. Combining crack with angst is one of my favorite ways to make this work (yes! pervert the canon for your own nefarious purposes!). Read it and weep with tears of laughter.
The Devil's Box (PG, Crossover with Reaper) by ivylecat. My husband's been nagging me forever to write a crossover for these two fandoms, but now someone's done it for me! FTW!
Supernatural Slash (Sam/Dean):
Across The Divide (NC-17) by pixel_0. A gorgeous, moody, heartbreaking story. Although written as part of the "Cock Thursday" series, that isn't the point or the effect of this story at all. A must-read (some S3 spoilers).
The Salt Of Your Skin (NC-17, Rimming) by cathybites. If you haven't already read this-- or seen the scorching hot (and Not Work Safe!) illustration here, then you are definitely missing out. This is all heat and urgency, and I think even those who normally don't find rimming all that hot will be persuaded by both the fic and the pic. Wow.