To Be Young Again by melodious329 NC-17 After a spell turns Dean into a much younger version of himself, Sam must learn to take care of his suspicious and now much younger brother. Sam learns a lot he never knew about Dean's early years and comes to a better understanding of the sacrifices his brother has made for him since he was a baby. This is a wonderful exploration of what Dean's life must have been like on the road with John, missing his mother and taking care of Sam, who never had to do without love or attention simply because his older brother was always there for him.
The sequel Coming Up Roses continues the story of Sam and Dean's changed relationship, chronicling Dean's difficulty in handling the new status quo.
Pushed Buttons, Crossed Wires by merepersiflage NC-17 (and kinky besides) This series of pornlets is still one of the hottest things I've read. A bit of pain!kink, some D/S overtones, and yet somehow still warm and loving, with the classic "Bitch" "Jerk" kind of banter.
You Hollow Out My Hungry Eyes by merihn (PerfectbyNature) NC-17 Half of all the fics from the first few seasons seem to feature some version of the magical sexual compulsion (fuck-or-die) trope. This was my introduction to the genre, and it definitely has held up over time. Sam is hit with bad reaction to a spell when he ill-advisedly uses a witch's own candle to bind her powers. Dean tries to withstand the pull, but even Bobby can tell it's hopeless. After all, it's Sam.
Down to the End verse by esorlehcar NC-17 Love this like crazy. Schmoopy, sexy, angsty deliciousness - Sam and Dean fall into each other like it's inevitable. Dean freaks out (probably also inevitable) and runs. Sam uses his powers and Missouri's help to track him down. The scene where Sam decides to tattoo a location charm onto Dean (and himself, fair is fair) so he can never lose track of his brother again is pure gold. Both men's desperation and desire is beautifully rendered, and the confrontations between the two, each trying to protect the other, only demonstrate how much better off they are together. Now if only Dad doesn't freak out.
Learning to Breathe verse by jassy3399 (Jassy) The one where Sam grows up to be a master thief and grifter. When Dean rejects 15-year-old Sam's advances, refusing to 'damage' his brother by giving in to his 'unnatural' feelings, Sam feels he has no other choice but to leave and make his own way in the world. Years later, the brothers reunite on a job and the tensions once again run high. But Sam's a different person now, and he's done running away.
I also love her Regrets verse, where Sam, miserable and desperate to stop Dean's deal, manages to go back in time and re-live his life over again. He saves his dad and keeps his brother from going to hell, but Dean still meets Cassie, and Sam can't let him give up on love. Even as he tries to give Dean space, new trauma awaits Sam - it seems that Azazel's army is still out there, and Sam will need all his powers to stop others from finishing what Yellow Eyes started. Featuring hurt!BAMF!Sam, the Roadhouse crew, and a bunch of the special-powers kids.
I'd Gladly Lose Me to Find You by flawedamythyst NC-17 Sam makes his own Deal to keep Dean out of hell - resulting in a vow of silence. The way Dean contends with Sam's lost voice, first anger and frustration, and then eventually acceptance has parallels in the developing bond between the brothers. Sam needs Dean like never before and Dean responds with unswerving, if sometimes bewildered, devotion.
Her Step by Step verse AKA the 'dildo verse' is a little less fraught - just some semi-angsty pornlets where Dean has to work his way through the reasons he and Sam are necessary to each other.
All We Have verse by pissedoffeskimo NC-17 This is still on the Sam/Dean Archive. It's one of the first fics I ever read and really hits a lot of the themes that thread throughout a lot of early-season SPN fic. Dad's disappeared and young Dean is left with no money and no way to take care of his Sammy - so he does what he has to for the money they need. When Sam is taken by vampires, Dean finally asks for help. He and Sam end up living at the Roadhouse (and sharing a bed). But now Dean has secrets he doesn't want to share and, when Dad does show back up again, a lot of pent-up anger.
Outside By the Blue, Blue Moon by ninhursag at vaingirlfic NC-17 (Non-con, dub-con, D/S, very kinky kink) When Sam, Dean and John are all caught in an evil trap, Sam sacrifices himself to the creature's dark desires in order to free his father and brother. After he's released, he finds his nightmare is just beginning - he's unable to speak of the spell that compels him seek out ever-greater depths of submissive pain and humiliation. With Jess he's able to find a kind of peace, but after her death his behavior leads Dean to ask some hard questions. The answers, along with Dean's willingness to do anything for his brother, may just help him put an end to years of torment.
Memories of Me by gillian NC-17 Dean gets amnesia and doesn't believe that he and Sam are just brothers. He can't figure out why Sam is putting him on, but he knows what he wants. Strangely, it's Sam who keeps everthing from falling apart when they get Dean's memory back. Excellent, excellent series.
Metallic Finish by lexalot NC-17 Smoking hot PWP. They're out at a bar and Sam's had enough. Parking lot Impala!sex. (Another favorite is Intangible where Sam's powers turn out to be really good for sexing up your snarky older brother.)
Preset verse by runawaydreamer NC-17 (RPS, kink, threesome) Dean meets Jensen, Jensen and Dean hook up. Sam and Dean have issues - the first time Jensen meets Sam, he's possessed by a demon. Eventually Jensen, in tandem with his boyfriend Jared, convince Dean to work things out with his brother. Somehow all of these things involve lots of delightful, kinky, schmoopy sex. Yay!
What Might Have Been also by jassy3399 NC-17 PWP Sam leaves the bar forlorn and alone, sure his brother doesn't want him around - Dean's got Cassie and Jo for company after all. When Dean gets back earlier than expected, they're both in for a pleasant surprise.
Walking in the Shadows by tempestquill NC-17 Dean lost his kid brother years ago. So when he stands up for an under age hooker in a bar one night, he's not expecting anything other than to help out someone who's had a rough time of it. But the more he learns about Sam, the more things fall into place, and the conclusions he's drawing are startling. Meanwhile Sam is determined to have Dean any way he can - and Sam's not nearly as vulnerable as he seems.
Things My Brother Taught Me by deannaz (Hellskitten) NC-17 Fantastic schmoopy brothers-together-forever fic series. Sam and Dean were Sam'n'Dean long before Sam went off to college. Now that they're back together on the hunt, Dean feels guilty that he can't keep his promise to his father to stay away from Sam. But not guilty enough to stop. Lovely illumination of a long, shared history of warm affection and physical bonding. Sam and Dean so obviously belong only to each other.
Venom series by audrarose NC-17 Dean in eyeliner, in a vampire club: can you say bad idea?(but delicious!)S This fic has a lot in common with leonidaslion's Pthonus verse, but I read this one first! Also, this one has an ending, where powers!Sam is determined to get his brother away from the head vampire, even at the cost of his own life.
The Hardest Heart by charlotte and brie at saucyminxfix NC-17 I first read this over on FF.net, forgot where I'd seen it, and then searched for it for weeks - and I'm glad I did. This is another angst-soaked story where the boys are split up early on - Dad catches Dean with Sam and forces him out of their lives at gunpoint. Deceived by his father, Sam grows up hunting, thinking his brother willingly abandoned him. Searching for Sam, but lacking the skills he'd have learned at his father's elbow, Dean becomes - a college professor(!) teaching folklore and mythology. Guess what happens when cynical hunter Sam shows up in his brother's office for research?
Dean's Massage and the EverChanging Spirit Oil by ilikecrystalsNC-17 Another fic I followed over from FF.net, the magical sexual compulsion trope strikes again - but this time the resulting fic features one of the best descriptions of a sensual massage I've ever read. Plus first-time sexytimes!
Issue of Consent by varkelton NC-17 Sam is (once again) under a curse that transforms his need for his brother into something darker. Can their relationship withstand the lengths Dean must go to in order to keep Sam safe and somewhat sane? This verse is often dark, but the brother's dedication to each other is made explicitly clear. A full immersion fic: once you start, you won't get your head up out of it until it's done.
What a Long, Strange Trip It's Been verse by mass_hipgnosis NC-17 Sam and Dean go to Vegas, work a case and get into trouble. Also, Sam can't keep himself from thinking bad, bad thoughts about his brother. He feels guilty enough, but still he sometimes feels like Dean's just trying to seduce him. What's a mixed-up younger brother to do?(I love Broken English, too. Sam and Dean have their own special language and they don't need anyone but each other.)
This is by no means intended as a definitive beginner's guide to SPN fic - just one beginner's starting point - after all, nothing's as sweet as love at first read ♥ ♥ ♥!