It all started with a very gifted boy... (or ended. Or middled).
Nihil Inherit by ratherastory (Gen, G)
Summary: Whatever you do, you will always end up... here.
In May of 2007, Sam Winchester wakes up in his own grave, but digging himself out is the least of his worries. He's plagued by the psychic visions which have returned with renewed intensity and which all but threaten to cripple him. He reunites with Dean, only to find that the Yellow-Eyed Demon is still at large, about to put his world-ending plans into motion. A mysterious figure who know much more than he ought dogs Sam's footsteps, hinting at darker things to come. Soon Sam and Dean find themselves on a path that feels frighteningly familiar: toward Lilith, Lucifer, and the Apocalypse. Unable to shake the certainty that he's seen all of this before, Sam knows that this is his one chance to set things right, or die trying.
Secure the Blessing by Rahmi (Gen, M)
Summary: Sam makes a wish. Dean is not pleased. (pre-WIAWSNB)
Who Here Would Choose (To Walk In Those Shoes) by philalethia (Sam/Dean, NC-17)
Summary: "I want to know how my brother dies," Sam says. To the wrong person. S3.
The Fire I Breathe by dreamlittleyo (Sam/Dean, NC-17)
Summary: What if Devil's Trap had ended a little bit differently? The YED's plan backfires, and Sam's got new problems. Because power like that, once you touch it you can't put it back. (And what can they do when John Winchester learns too much?)
Old Country by astolat (Sam/Dean, NC-17)
Summary: Sam and Dean go to Hogwarts. (spoilers for All Hell Breaks Loose, Deathly Hallows)
Specific Relativity by gekizetsu (Gen, PG-13)
Summary: an AU Sam who has always had visions and is not quite right as a result. Don’t worry; he’s mostly happy. Casefic; notquiteright!Sam and superprotective!Dean hunt for the ‘special children’ gone bad when they’re not hunting demons with their father. Cuddling, mild angst, cuddling, humor, cuddling.
Captured by the Game by rivkat (Sam/Dean, NC-17)
Summary: AU. Azazel has given his favorite son a task: worm his way into the confidence of a hunter. It sounds simple, but Dean Winchester just might be more than Sam can handle.
The Hunter Games by theproblematique (Sam/Dean, NC-17)
Summary: When the infamous Winchester bad luck strikes twice in quick succession Sam and Dean are forced to compete in the most brutal reality TV show ever created. It’s impossible to escape the battlefield, hiding can only be temporary, and alliances inside those dark, bloodstained woods last about as long as it takes for the other Hunter to figure out how to use your weapons. And then kill you with them.
Down to the End ‘Verse by esorlehcar (Sam/Dean, NC-17)
Summary: "I'm psychic, you stupid son of a bitch," he says.
Rec Note: (entire ‘verse set in early seasons, beautiful and the best of Sam and Dean)
Turn of the Wheel ‘verse by gekizetsu (Sam/Dean, NC-17)
Summary: Humor, wingfic. Dean is bitchslapped by an elemental and it's Sam who has to cope.
Rec Note: Dean is the one with wings, Sam develops powers of his own later on. One of the most compelling lore fics I’ve ever read.
The Touch of His Hand by macbyrne (Sam/Dean, NC-17)
Summary: Sam is fourteen when his psychic ability manifests, in the form of powerful, uncontrollable visions that leave him incapacitated by pain. Fear of his father’s reaction to having a son with supernatural powers leaves him only one person to turn to: his brother Dean. As his feelings for Dean grow stronger and the visions take more out of him, both brothers will have to make decisions on what is important, and where you go when you have nowhere left to turn. (pre-series)
Unasked by astolat (Sam/Dean, NC-17)
Summary: Sam doesn't ask.
In Florida, when they burst out of the old collapsing mausoleum into afternoon sunshine, covered with dirt and ectoplasm and victory, and Dean punches his fists in the air and looks at him, dirty and grinning and shining so bright that for a minute Sam almost can't see, still he doesn't ask, he doesn't reach out and put his hands on Dean's face, where they belong according to every instinct and yearning he has in him.
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There are many amazing powers!Sam fics out there so please feel free to share your favourites :)