In Japan there is basically one forum for all purposes and it is 2-channel. It's enormous, anonymous, and text-only (aside from the porn ads). There are hundreds of different boards on all kinds of topics. You can spend days getting lost in it…
As far as I remember, it was originally a hastily written message board for a television channel web site in the mid-1990s but quickly grew out of control. The site was then moved and disassociated with the network and became its own entity. It developed a culture all its own and now reigns supreme as the place to be online. It is largely anonymous.
Things that have sprung out of it include a few whistle blower scandals and the movie Densha Otoko. There's a huge cast of characters in the ascii art world too which I'll get to some day…
UPDATE: As of 21 September 2015, the founder of 2ch now owns 4chan.