You may have seen some of these, but I tried to get the more obscure ones. I'll update this page as I find more, or as stuff gets taken down. You'll have to copy paste the links, I can't get links to sites off neocities to work for some reason.
Erowid.org is a giant library of psychoactive substances. It has tutorials for safely synthesizing and using pretty much anything you can think of. Is it irresponsible to post a guides for how to cook meth? Who cares, it's fun! It also has a forum for trip experiences. Some of the stories are pretty disturbing so be careful what you click on.
Asoftmurmur.com is basically a custom soundcscape builder.
superbad.com is a super creepy website. Has a bunch of weird minigames, will probably get you put on a watchlist. 10/10
Neal.fun has a bunch of fun games. My favorites are Stimulation Clicker and Absurd Trolley Problems.
Dood.al/pinktrombone is odd, which you may have guessed based on the name. It's a simulation of a human mouth and voicebox you can squish around with your mouse. It's fun to try to get it to talk.
973-eht-namuh-973.com reminds me of superbad.com. Lots of creepy religious stuff.
16personalities.com is that giant personality quiz people love to turn into their entire personalities. (I feel like there's a joke here but I'm not smart enough to figure it out)
Idrlabs.com also has lots of quizzes, but most of them are just for fun.
Heavensgate.com is still up, surprisingly. It's a fun read, but you might get put on a list.
kiwifarms.st has a ton of very odd forums. I didn't stay for long, but it seems entertaining.
lhohq.blogspot.com is a crazy, very unsettling feverdream.
libraryofbabel.info is based on a short story about an infinite library. Basically, the idea is that if you randomly generate letters in strings over an infinite amount of time, every possible combination of letters and spaces will eventually give you 'hidden knowledge'. It's probably bullshit, but the website is fun to browse.