I've barely begun to explore the Harry Potter fandom, so any one who wants to recommend to me their favorite Potter fic or fanfic site, please do so. I read slash pretty much exclusively, and prefer Harry/Snape or Harry/Draco.
Lest I Wither by JiM
Harry Potter
I was a little leary at first of reading any Harry Potter, mostly because I have objections to underage-fic, but I discovered some really well written pieces, starting with the "Mirror" series and "Lest I Wither", both of which feature a grown-up Harry, and both of which are awesome stories.
Potter is poisoned and Snapes must find the antidote. This is an excellent introduction to Potter fanfic.
Mirror of Maybe by Midnight Blue
This is an utterly awesome story, spoiled only by the fact that it's NOT DONE! If I was rich I'd hire the author to finish it.
Underwater Light by Maya
This is a marvelous blend of rivalry and the growth of mutual respect between Harry and Draco. The humor in it is delightful; it had me positively rolling in spots. Very sweet, like the taste of wild honey. Still WIP, but the author is currently working on it, thank god...
The Lodger by Mad Martha
After the War is over a weary, slightly bitter Harry looks for a lodger, and ends up with Draco, a haunted outcast in the wizard world even after he testified against his father. Very sweet.
Commencement by Aja
On the eve of graduation, Harry and Draco get drunk together. Moody and sweet and sexy.
Lustre by Calico and Julad
Draco is given a beautiful snake by his father, and he must turn to Harry for help when it becomes ill. Very nicely told tale.
For Services Rendered by Minx
Snape needs the services of a Parselmouth, and Harry bargains with him for sex.
Naked Quidditch Match You'll laugh until you cry at this series of emails between the various participants in this little comedy when Fred and George lose a bet, forcing the Griffindor team to play a game in the buff.
Growing Up by Devi the Wynter Wytch
Potter is dared by the Weasleys to proposition Snape. Snape has had enough.
Rite of Passage by Rushlight
Potter has to choose someone to initiate him into adulthood, and he picks Snape. The idea of it bothers me, but it was so well written that I found I enjoyed the story very much.
If You Are Prepared A recent find, a very well told and substantial story, deals with the underage things well.
Individual Author's Pages for Harry Potter fiction
HP Yaoi Slash Includes fanfic by Devi the Wynter Wytch, Duo Lord of Death, Keara, Mana Crystal, Mcee and Myr.
Other Harry Potter links
And here's the Google entrance to alt.fan.harry-potter, the Harry Potter newsgroup. This is the definitive archive for Harry Potter fanfic. It is accessed by a beautifully maintained archive page.
Malfor Slash Archives
Lady Kardasi's Domain Home of the Fuh-Q-Fest, features fic from Sentinel, Smallville, Star Trek, Highlander, Due South, X-files, Buffy, Stargate, Lord of the Rings and Harry Potter.
The Gryffindor Tower Calls itself "A Harry Potter fanfiction archive for the swashbuckling pirate"
Red Rose Press
Temporary Insanity "Pop-slash" by Calico and Julad and Nemoinis
Potter Slash Archive
Slash City
Snape Fuh_Q_Fest
Pirates of the Carribean
Woo-woo! I just saw Pirates, and boy, is Cap'n Jack ever hot. Very slashable, and he'd even be good with the ladies. Maybe. Never mind...back to the idea of slashing Captain Jack Sparrow...
Very hot little story, takes place right at the end of the movie. Slightly different ending.
Other fanfic
Tayla's page
Fanfic from CSI, Dead Zone, Invisible Man, Looney Tunes, Nash Bridges, Now and Again, Sentinal, Simon and Simon, Stargate SG-1, Starsky and Hitch and some original fiction
Other stuff
Fanfiction.net Can't say as I've been terribly impressed with the quality of the fanfic I've found here, but I get the impression that the writers aren't taking it particularly seriously.
Vampire Hunter D Archives Mixed quality, nothing spectacular, a few fairly erotic pieces
101 Ways ...to end up in a Canadian Shack.
Sorka's Slash Includes a Sentinal/xfiles crossover called the Inquisitioner.
Juxian Tang's links All the slashy archives you can shake a cock, er, stick at...
Minotaur's page Tips for slash writers by a "thirty-something gay man, who's lived basically his entire life in the gay community"
How To Write Fanfic Humorous piece by someone who is obviosly not fond of Badfic.