two actually..
First one was a place called Ayenee in Yahoo chats. Was fun, lots of "Roleplay" going on back in the mid 90s. was super popular for awhile then Yahoo changed their chatrooms and killed all user created chat rooms instantly.. It was actualy my first "MMO like" experience. Being in a world with thousands of others, making friends, going on adventures etc.,
City of Heroes.
Still boggles my mind how NCsoft could not figure out how, in the superhero boom that was taking place, to monetize it better! no reason they should have killed it off, for freaking POS Wildstar of all games. They won't sell the IP to anyone else (Lots of developers/studios tried to buy it). And now they are in the crapper.
I won't say Everquest because it still has a sizable community playing on it. Heck I play on it once a week or so just for the feels and remembering being a teen thinking HOLY CRAP this is AMAZING
I was also part of a huge Vampire:TM forum which died. At one point it had 20k registered people.