I have a whole list of Buffyverse recs here.
Better Buffy Fiction Archive
The mother of all rec lists.
This is, last I checked, a comprehensive and up-to-date listing of Buffyverse ficathons.
BtVS Episode Guides and Transcripts ~and~ AtS Episode Guides and Transcripts
These are absolutely the most useful thing ever.
Buffy Dialogue Database
This looks like an insanely huge labour of love, and a fantastic resource for anyone writing about the worlds of BtVS and AtS.
The Annotated Buffy
It's not complete, and who knows if it ever will be, but what's up there is pretty darn cool.
Once A Thief
The Agency is a gorgeous site devoted to Once A Thief. It includes the most comprehensive fiction archive I know of, plus fan art, an episode guide, writers' resources, and cast and character guides.
This Once A Thief slash-centric mailing list isn't very active anymore, but it still has archives dating back to September, 2000. And hey, lots of people still subscribe, and you could always come by and spark a flurry of activity with your fresh enthusiasm!
Once A Thief LJ Community
Or, if you prefer LJ to mailing lists (or want to have it both ways), go here.
For a while now, I've been periodically posting challenges to the RatBoat list. Here's a list of all of them, plus links to everything that's been written in response.
That '70s Show
That '70s Fanfic Archive
This is the best archive I know of dedicated entirely to That '70s Show. The fiction is grouped by category, making it easy to find what you're looking for.
The Fotohut is a fan site dedicated to both the character Hyde from That '70s Show, and Danny Masterson, the actor who plays him. There are lots of wonderful pictures!
Fan Forum: That '70s Show
This is where the most lively meta and squeeing discussion is going on, last I checked.
due South Fiction Archive
This site took up the torch years ago from Hexwood, making it the central archive of Due South fanfic. It's got a great search engine (lets you search by pairing, warning, author and more!) and it's still being updated frequently with new fic.
General Useful Stuff
Erowid Experience Vaults
This is a collection of people's first-hand experiences of doing various drugs. It's quality-controlled, so there aren't any simple "Whoah, it was awesome, dude" entries—each testimonial gets specific about what the high or trip or whatever actually felt like. For someone like me, who writes characters who use drugs but has no intention of trying them herself, this is awfully useful.
Is there any point in linking to this? Is there anyone in the fanfic world who doesn't already have the simple URL memorized? Well, once upon a time when I was a newbie writer, I didn't know where it was. My confusion was compounded by the fact that people tended to refer to it (affectionately?) as ff.net, but if you actually go to www.ff.net, it's something entirely different. Anyway. This is the uber-archive. If you've ever wondered "Hey, has anyone ever written something in Random Fandom X?" this would be the first place to look. It has many, many weaknesses, including annoying pop-ups and the fact that it isn't hard-core porn friendly. (And what's fanfic without the occasional hard-core porn?) Truly, 99% of it is crap. It's the place 13-year-olds go to post their very first Mary Sues. It's the place where bad spelling goes to party. Just reading the story summaries on any given fandom index page could make you cry. Grit your teeth and repeat to yourself: "Diamond in the rough." Some good writers post there too, and some (especially new ones) post nowhere else.
More links to come later. If you know a site I should link to, please let me know! And if you find a dead link, please be kind and email me.