Slayer's Sanctuary: This one seems to be universally admired by cyber Buffy fans. And I hate to go along with everyone else but a well-kept website is a well-kept website. Its best features are the abundance of links (arranged by character so it's quite easy to find Sarah Michelle Gellar pages or Anthony Stewart Head pages) and the archive of magazine and newspaper articles.
Buffy the Vampire Slayer: the Music: What else is there to say about a page that not only prides itself on knowing what song is playing as Willow walks past Oz's van but also tells you where you can get it other than that Leslie, the site creator, is a genius and she has a little too much time on her hands? Cross-referenced by show, artist and song.
The Sounds of the Slayer: Buffy is Clueless meets Salem's Lot and never is this so apparent as in the funny things the characters say and the way they say it. Find here dozens and dozens of sound files from the show; arranged by episode.
The Slayer Fanfic Archive: This site contains transcripts from the entire first season and part of the second. There's also tons and tons of fanfic, for which Anya has very nicely provided author and story indexes. I haven't read much of the stuff so I don't know if it's good, but I am encouraged by "After the Dance" by E.T. Hills, a story that works in Pike from the movie.
The NEW Buffy the Vampire Slayer Tape Trade: The page propietor and address are new, but the original mission remains the same: "Those seeking specific episodes of BtVS can locate those who have the needed episodes quickly without bugging people on the mailing lists." Sounds good to me.
The Little Buffy Site: If nothing else, the illustration on six-year-old Buffy's T-shirt--that of a fanged, evil-looking smiley face with a red line through it--earned my eternal devotion. This site has lots of songs and top-ten lists and other stuff that speculates what it was like to be a toddler and a vampire slayer. Alas, the tired "slay" puns, which do get to you after a while, caused me to question the meaning of "eternal."
Buffy the Vampire Slayer Humor Page!: This is a really cool page. It's interactive and funky. It's the place to go to play episode-related mad libs. It's where neat bloopers and continuity snafus are caught. It's the home of the -ism--Buffyisms, Xanderisms, Willowism, otherisms as well as fun Buffy quotes. It's unique and interesting and worthy of a visit or two.
Sunnydale High School Library: This is an excellent site for those wishing to expand their knowledge of vampires and other icky things. It's got links to the Canadian Chapter of the Transylvanian Society of Dracula and the online Encyclopedia Mythica. Then, of course, there is some Giles appreciation stuff going on. That's not quite as interesting.
Punch's Buffy the Vampire Slayer Page: Contains the usual assortment of cast bios, pics and links. The last are color coded by characters so that all Buffy links are blue. This is neat but not as cool as the Graveyard page, which keeps track of all of Sunnydale's dead. Punch has provided pictures (where available) and descriptions of where they fit into the plotline. Personal fave: Herbert the pig, former mascot who was eaten by Xander and four other students. Very good use of blue.
Buffy the Vampire Slayer: The official site offers some good stuff. There is the "slayspeak" section that defines show terms like "astral projection" and "vampire," and the "bronze" room and posting board that lets fans chat with other fans. Frequently asked questions are answered (though these are pretty basic like "Buffy, why her?"). Lots of great photos, especially of the monsters. And don't miss the postcards. They really are the best thing about the site.