Physics Forums, which is a site for research level mathematicians and physicists. It's a bit above me, but can be useful.
Bluelight, which is a site giving guidance on responsible drug use.
The Student Room, which is a message board for students. It's a bit rubbish mainly, but sometimes has something interesting.
Something Awful, which is a general discussion and comedy forum.
A couple of others, which I either don't want to mention or don't frequent enough to be worth mentioning.