Affliction, by Tolakasa.
Gen, Teen, 45k words.
Author Summary: AU of season 5, veering off from canon after the end of 5.04, after Sam and Dean are hunting together again. In a diner one day, Sam notices that something's missing. Something kind of important.
Why I'm reccing it: It is excruciatingly well-written and the characters are all true to the show and in tune even under the circumstances. The story unfolds in a delightfully agonizing way, and it ends where it needs to end rather than anywhere else (don't want to spoil anything!). The central plot reveal caught me 100% flat-footed, but I saw the clues in retrospect, major props for that. It's one of the few things to make me smile, gasp aloud, giggle and drop my jaw in amazement during a horrible week.
If you enjoy reading Supernatural Gen fic, this Season-5-diverging AU has a lot going for it and is well worth a look. An absorbing read.