>>2695
https://www.2chan.net/ , and there's also this fan-translated homepage: https://futabachannel.neocities.org/
If you're after some other active 掲示板 and 画像掲示板 sites, here's some of the more popular ones:
Textboards:
5channel - https://5ch.net/ (formerly known as 2channel; European IPs are blocked so you'll need a VPN to browse it)
Bakusai - https://bakusai.com/
Machi-BBS - https://machi.to/
Shitaraba - https://rentalbbs.shitaraba.com/ (formerly Livedoor)
Imageboards:
Futaba Channel - https://www.2chan.net/ (mostly 2D-orientated: anime, manga, games, etc.)
Momizou - http://momi3.net/ (mostly 3D-orientated: TV, idols, models, gravure, etc.; a continuation of former imageboard Momiage Channel, which is where all the Futaba refugees went when they closed their TV boards in 2007)
Momicha - https://momicha.net/index.htm (a 2018 offshoot of Momizou due to concerns with overbearing moderation)
Meiwa Suisan - http://www.meiwasuisan.com/ (probably the least "otaku" of these imageboards; more orientated towards news and general interests)
Bit of Japanese imageboard trivia for you: While traversing Japanese imageboards, there's a good chance you'll happen across drawings of a certain character (and related screencaps/copypasta) being spammed everywhere. The character is usually grotesque, fat, middle-aged, wall-eyed, buck-toothed, wears a nappy, has a South Korean flag on his forehead, is balding with water squirting out of his head, has large protruding nipples, and has amputated legs. He's often seen being the recipient of abuse from various other characters, killing himself, or being pampered like a baby.
Apparently it's a piss-take of a notorious Japanese lolcow/spammer/troll/paedo, lovingly nicknamed "オムツァー丸の内武" (rough TL: nappy-shitting bloke from Marunouchi), who's supposedly been shitting up Japanese imageboards for the past 2 decades, has a very distinctive way of posting, and often posts images of Showa-era idol Naoko Kawai everywhere. Some claim he's shat some places up with CP, scat, and other shit to the point where they've had to close certain boards or even shut down entirely, but I don't know how true that is. In recent years, some Japanese anons have taken to counter-trolling him by spamming caricatures, incriminating screencaps, and humourous image macros wherever he's known to frequent in order to rile him up, as he supposedly cannot handle criticism and is generally fun to ridicule.