420chan is an imageboard based on cannabis culture but has expanded far beyond just cannabis. This one has survived since 2005 and is still active today.
Marking as dead 7-21-22 – loads to cloudflares offline cached page. Also from this link it sounds like the admin was doing sketchy stuff. Feel free to comment below if you have more accurate info.
Bought by Copypaste / Fredrick Brennan, site is under construction and will probably take a long time.