Something Awful forums -- require a one-time $10 fee to get in

Fark -- used to be well-known for their photoshop contests...

YTMND -- lots of in-jokes, but at least they're more accessible (and documented too!  http://wiki.ytmnd.com/)





==== Imageboards ====
- 4chan
    /b/ is the most well-known one, and sometimes the most bizarre
    /d/ (Hentai/Alternative) was most enlightening, to me at least
    /gif/ (animated gifs) is also good
- others: 7chan, ...





==== more minor ====
Stile Project -- more porn and shock-site stuff