Futaba Channel (or confusingly, 2chan) is Japan's most popular Imageboard. Users with IP addresses that can't be traced back to Japan cannot post.
Futaba Channel was extremely popular on Something Awful's ADTRW boards, as it's anonymous image posting system was important in an era before image hosting. The site inspired moot to create 4chan, and it's code, Futalullaby, formed the basis of 4chan's Yotsuba codebase (though by now it has been heavily rewritten).
Futaba is home to epic raids that you can't participate in.