The BTVS show was enormously popular in geekdom and gothdom, so much so that thousands of Buffy (and later, Angel) websites sprung up. There were fan fiction, slash, crossovers, Buffy tarot deck art contests, webrings, fanlistings for every possible episode and combination of characters, and thousands of episode guides.
Not personally a huge fan of the show, I knew it was an important resource. Megafans would already know where all the cool Buffy sites were, so the point of my links page was just to help out moderate fans who maybe didn't have time to sit around on Altavista and Hotbot all day long (at the time) looking for Buffy stuff.
I tried hard to sift through the dreck and present good quality Buffy sites, but I also wanted completeness so added sites that I liked which weren't huge megaresources but still had nice things to offer visitors. Checking and maintaining the Buffy-related links was one of the most difficult chores that Dark Side of the Net presented me with.
I once did a Netcraft search for domain names with the word "buffy" in them, thinking (mostly correctly) that webpages which bothered to pay for and register their own domain were probably better quality and more likely to be maintained than Geocities type sites. Wow, that was a week or so of research and looking through the findings.. Fun, but a bit time consuming!
A thorough episode guide, plus a link database, gallery, and other offerings. The site is no longer being updated, but it's left up because the episode guide is still a great reference.
2. Buffy & Angel Fan Community
A very active Livejournal community for BTVS fans.
I'm amazed that this Yahoo! group is still active. It's a serious scholarly group for the academic study of the Buffyverse and related school projects.
I don't know who the person is behind this extremely active and on-top-of-everything Twitter account, but they are doing an awesome job. If you care about Buffy or all things Whedonesque, you should follow this.
5. BTVS Fans
Another active and interesting Buffy LJ community. Not sure what else it might have to offer other than the above LJ community, but it's worth a look too.
A searchable database of every Buffy character, episode, cast member, writer and director and every word of every show. Wow! I imagine this was a ton of work.
7. Buffy Facebook Fan Page
Central meeting point for Buffy fans on FB.
8. Buffy Boards
Very active message forums for Buffy and Angel. There is even a board for Season 8, the graphic novel continuation.
9. Buffy the Vampire Slayer Fansite
I really have to hand it to these people, they are dedicated! This collaborative site is still going strong, and is lovingly tended by Buffy fans. There's not a ton of info here, but what's here is nicely organized, spelled correctly, and pretty up-to-date.
10. Buffy RPG
Forums for Buffy roleplayers all over the world.
Another extremely active message forum and community of Buffy, Angel and Whedon fans.
12. BTVS Figs
Here's a niche community of people who create Buffy-related figures. While this isn't a hobby I have, I had a lot of fun looking through their creations here.
13. Twisting the Hellmouth
This is a GIGANTIC archive of crossover fan fiction stories by different authors. Buffy, Faith, Dawn and the others have quite a lot of adventures here, meeting everyone from Batman to Princess Leia, Gandalf to the Cullens. I am impressed how cleanly designed and well organized the site is too. I'll definitely be reading a few stories here tonight before I retire.