http://pooshlmer.com/wakaba/wakaba.html Imageboard style, similar to 4chan/etc. quite popular and random, but may contain offensive content.
http://www.shrinemaiden.org/forum/ I believe that's a popular forum--then again I just lurk, so I wouldn't know; appears to be.
Luck. - Spikey00.
http://lunaticred.proboards.com/index.cgi Not that famous but a good and well-maintained forum with respectable members. It also hosts some tournament like UNL.