I see someone already posted http://www.everything2.com/ (my used-to-be favorite) and http://www.fusionanomaly.net/fusion.html .

Another older one: (somewhat NSFW horror stuff): http://deathandhell.com/

TBH my current rabbit holes are YouTube and repeatedly clicking "random" on http://www.smbc-comics.com/ .

edit: Another couple old haunts:

http://www.chaosmatrix.org/

https://qntm.org/