Japanese Imageboards
http://www.siokara.com/siobbs/
http://2ch.homelinux.com/
http://namamono.tk/
http://www.meiwasuisan.com/
http://www.bigbbs.net/
http://windy.sakura.ne.jp/nijiiro/
http://www.haruzine.com/index.php
English Boards
http://1chan.net/overchan/
I recommend Pixiv.
http://www.pixiv.net/
http://en.wikipedia.org/wiki/Pixiv
Pixiv Guide
http://danbooru.donmai.us/forum/show/4517
http://witch.animeblogger.net/2009/02/13/pixiv-registration/
http://teruchan.deviantart.com/art/How-to-Registration-pixiv-97389536
http://www.wildcritters.us/wc/forum/show/23
http://img413.imageshack.us/img413/4121/pixivreg01qy3.png