If you've managed to find my site without reading any of the following, I strongly recommend you give them a shot. I don't rec fics lightly, because I don't want this list to become unwieldy, so these are the favorites of my favorites. Some may require passwords to access - they're easy enough to get, and generally only involve an email confirmation that you're of age and understand what you're about to read.
Stories I recommend may include: underaged sex, bestiality, homosexual relationships, Non-consensual sex, BDSM, and other potentially touchy concepts. If one of all of these things is not to your tastes I suggest you read the warnings carefully and avoid those that may bother you. Please note that my recs here may not contain all of the warnings for a particlar fic. and don't contain the ratings; for this reason I strongly suggest you read the ratings/warnings in the headers to the fics themselves.
The Sensible Garden
By RaeWhit. A little pixie brings Harry and Snape together. I don't like kids in fic, just like I don't like poetry in fic. And yet RaeWhit and written stories with both in them that I have not only liked, but loved. The Sensible Garden is a lovely (and sad, be warned) romance. It's hard to describe, but has the same careful craftsmanship of all RaeWhit's works (so read others while you're there). The link goes to the author's homepage, click on the fanfiction link for a listing of stories.
Round Bottom, Twelve Inches
By Pen_and_Umbra. This might be the funniest fic I've ever read, Snape's voice is fantastic.
Regular, The Usual
By OpaqueVision. Snape is sent to retrieve Harry from muggle NY, but as usual in fic - things do not go as planned. It's hard to desribe why I love this fic... it's a very real fic.; it manages to be both sad and happy at the same time. Lovely prose are just the icing on the cake. Link takes you to a listing of the author's snarry stories, scroll down a bit for this one (or read through them till yo reach it).
So Lonely Without Me
by calligraphy, a beautiful, sweet fic. Snape and Harry are locked in a single room (plus bathroom) for an extended period of time. It's simple, and clean, despite the complex way in which she reveals her characters' psychologies through action and word, rather than narration. A must read for slash fans of excellent writing.
Severus Snape and the Pensieve Memories
By OpaqueVision. Voldemort forces Snape into forcing Harry. A wonderful character portrait of Snape, and the best interweaving of the story with canon that I've seen. This is a story for those of you who enjoy the art of writing, and subtlety. Warnings for non-con and chan.
By DementorDelta, Don't be discouraged by the semi-bestiality element - this fic is perfect if you're in the mood for something light, fluffy, and just a bit erotic. There's a touch of angst, but nothing severe. While you're at it, here's the link for the author's skyehawke page - go read all of her other stories too, especially 'Between the Lines' and 'Dumbledore's Folly'.
The Scent of Pines
By Kira-nerys, A Snape/Harry BDSM PWP that's... just plain hot. Harry's first experience, I believe he may be officially underaged in the fic, but it's easy enough to ignore that element and pretend he's just old enough...
A Nick in Time & Growing Pains
By Tira Nog, The fic that made me give all de-aged character fics a chance. Snape and Harry are children again, and they become friends. Coupled with it's sequel, Growing Pains, a Nick in Time goes from sweet/cute to angsty, to romantic/erotic. It's got all the bases covered and it's well done.
Something to Write on & Quill and Ink
By Cybele, BDSM themed, another set that's just plain hot. Link goes to the author's page at Walking the Plank.
Aucta Sinistra. I realized, while reccing some ss/hp fic to a new reader, that I've probably re-read this fic more than any others. It's very light and sweet, and I wish I could put into words why I love it so. The link takes you to the author's page, and it's linked from there. All of Aucta Sinistra's works are good, so read everything else she's posted while you're there.
Dark Side of Light
By Maya, Be Warned - This fic will Rip Your Heart Out. Then it will drop it on the floor and stomp on it, toss it into a vat of acid, and if anything's left it will garnish it with a sprig of parsley and have it for an afternoon snack. Draco's locked up, Harry is too but not in the same way. I'd love to say more, but I would give too much away. Exquisitely written. High squick warning.
Welcome to the Real World
By IAmtheLizardQueen, Once you've had your heart torn out by Dark Side of Light, get it repaired by this very sweet and well done fic. Draco finds himself sharing an apartment with Harry in the muggle world. It's scary, and new, and a good thing he has Harry to help him through it all.
RaeWhit, Harry/Draco, Harry/Severus. NC-17 see warnings on the author's page. 91,000 words and all of them good. This story is just beautifully crafted. It's real, and the emotions and relationships between the characters are complex to the point of uncertainty (in a good way, imho). Romantic and Believable.
The Shoebox Project
The Only Work In Progress I will recommend. This isn't a fanfic, it's a novel whose characters happen to share the same names as Rowling's Marauders. The author's voice is strong and unique, the characters so realistically fleshed out you can practically touch them, and Lady Jaida keeps us on the fine edge of UST.
By Klynie, Snape/Draco/Harry. In desperation Draco is forced to become the crux of a bond between Snape, Harry, and he. What can I say? I love Draco in this story, he's weak, and desperately trying not to show it.
A Not So Graceful Redemption
By Ships_Harry, Filch/Snape.
From the author's header: "Severus Snape and Argus Filch find some common ground one Christmas. Warnings: Snape is 17, and there's some rimming, plus reference to some bondage and corporal kink. Images are NOT WORKSAFE. " -- Don't let the pairing put you off - if you enjoy creativity and good writing, then read this piece. It's text and verse, and she manages the difficult task of getting emotion into a sing-song rhyme scheme.
Draco Malfoy the Amazing Bouncing...Rat?
By Maya, There are only two Het fics I've read thus far that I will recommend and this is one of them. It's Draco/Hermione, but really the romance is secondary. The whole point is to enjoy the vastly entertaining version of Draco that she's written as he struggles to free himself from his Rat-shaped affliction. Also readable at schnoogle here.
The Fire and the Rose
Abby & Domina, Snape/Hermione, NC-17. It's a cliche body-switching plot but, frankly, I don't see anything wrong with a plot device like that when it's in a universe where it's theoretically plausible. As long as it, like this one, is done well, be as cliche as you want. Abby and Domina build the story slowly (it's forty chapters, so be set for a long read) and it's (in my humble opinion) realistic/in character for such a situation. Don't really know what more to say - give it a shot. I'm not a Hermione fan at all and I very much enjoyed this story. There's a continuation, but I can't speak for it as I haven't read it.
The Snarry Reader
Like the title suggests, a listing of recced Snape/Harry stories.
Painless_J's Thematic Master Lists
In the mood for a particular kind of fic? Try here. It's a valuable resource for many reasons, not the least of which is that you can find new pairings by selecting a type of fic you enjoy and then trying out a new pairing in that category. Also a handy resource when you vaguely recall what a fic was about, or what kink it contained, but not title, author, or where you found it...
List of Lists
Another masterlist of themed lists, like and including Painless_J's.
Ink Stained Fingers - A Harry Potter fanfiction archive that's mostly slash. There are, of course, many, many other archives, but this is one of my personal favorites because the updates are quick to get and the pages easy to read. It has most of the older classic stories on it.
Saturn's Harry Potter Recs. - Lists many of the stories I would put here, if I wanted a longer recs. list. Including some that aren't on the major archives.