Asklemmy

There's a lot - MSN, forums in their hey-day, Geocities, the days when almost every ISP had free web hosting, long-form letter style emails being a common means of communication, email newsletters, etc. etc.

But three particular things come to mind:

R.E. League at Reality's End (https://www.realitysend.com/) - like many children in the 90s-2000s, I was obsessed with Pokemon. This guy made a web-based Pokemon RPG, with all the usual stuff (battles, badges, etc.) BUT with puzzles and amazing writing. What's more, anyone could make a website and be their own gym leader, WITH BADGE, with just a little code from Reality's End. It. Was. Awesome. Still around, kind of, but most of the fan-made gyms are looong gone, and I'm not certain you can create a new user for the main site stuff anymore.

[Town Redacted]MP3.com - As a young teenager I lived in a small city with a disproportionally lively music scene. This website had recordings from local artists of all genres, and started me off on finding all ages shows [this was also still thriving, I don't see that many posters etc. for these anymore]. Met a ton of other kids, listened to a lot of good music, and it became a 'cool kid' feather in my cap. Also, in a roundabout way, was a catalyst for me discovering pot (listen to music and click on link for the band's webpage -> get into a discussion in their guestbook with a member of a different, all teen girl punk band -> start chatting with members of this band often -> band smokes me out for the first time at a show for the band we met over, which was also my last night in town before moving)

The Pojo.com Forums - Again, Pokemon obsessed kid. Loved Pojo for TCG stuff, but it also introduced me to fanfiction. I wrote my own fanfics that were well received, which gave me a lot of confidence about writing in general. Also met some cool people here, and for nostalgia's sake I find myself Googling usernames I remember sometimes.