Pylean Refugee which is all Fred, all the time. Heh heh heh.
Potential: A Celebration of Kennedy is a website devoted to our favorite red-haired witch's new honey.
Your Mission... is an archive of fanfic challenges. Cool, huh?
Plan C is a message board (and the story behind the name is a dilly, btw) about--what else?--the Buffyverse. Really fun.
Wyndgyrl's Buffy FemSlash is a really fun website, its content pretty much self-explanatory. Includes a really good Faith/Dawn series.
Through The Looking Glass is a website filled with x-over fic. Willow/Tara in Starfleet, as super-heroes, battling Orcs, etc.
Better Buffy FanFics Archive is an earnest attempt to gather the equivalent of a "Book Club" for fanfic. I strongly recommend you take a peek.
The Tao of the World and Beyond is the website of the wonderously fun and talented Kieli. Check it out. Really. She's a marvelous lady and a good friend.
Willow & Tara's Magic is another of many, many W/T sites on the web.
A Suave, Not-Very-Pathetic Slayerette Site is a site that has had its share of troubles (i.e. viruses in the webmaster's computer) but seems back on track. Yay!
Adult Willow Fan Fiction Archive pretty much says it all in the title.
Willow's Lil Secret is a fun little site, put together by a really dedicated fan. Check it out!

Buffy RPG Want Ads is where players of Buffy RPGs advertise for players and such. Its fun.

Shippers United is a cool message board of various Buffy fans who hang out together.
A Darker Shade of Pale is the place for angst-ridden fan-fic in the Buffyverse.

The BTVS Writers' Guild may not be the ultimate Buffy fanfic site on the web. Then again, maybe it is...
The (Erotic) Adventures of Buffy and Evil Vampire Willow is a comic strip that's a hoot! (WARNING: This strip is rated "R")
Wiccan Ways! is a wonderful site full of great fic.

Rancour's Review of Never The Twain? is pretty much what it says it is. And I'm very pleased with that fact (not to mention Rancour's kind words...).
Citizens Against Bad Slash is a fun site full of opinions that are likely to get you thinking at least (not that that's a bad thing...).

BuffySearch.com: Your Portal to the Buffy/Angel Community The Paradoxical F/F Archive contains lots of femslash fanfiction, much of it adult, and in various fandoms. Mostly Trek but plenty of Buffy as well.
All Mine is a fanfic series thats very intense--about the pairing of Faith and Drusilla! Not for the faint of heart.
Dread Pirate Emily is a really fun site that includes a fine tribute to Buffy.

Extra Flamey is for all practical purposes the site for Willow/Tara fanfiction on the web.
Destroyer of Worlds is a Spike/Buffy site. An intriguing premise, and the site has lots and lots and lots of fanfic!
Faith Rocks is a really cool site devoted to the raven-haired Slayer who's seen the world from both ethical sides.
Kleysha's Bad Girls hasn't been updated in a long while, but as an adult archive its still one of the best.