I've only read mumsnet once or twice and it worries me, I think it was a thread on how complicated driving is that got to me. I don't read it any more, I like to pretend that I'm not sharing the roads with such scary people who have so little control over their cars.
I used to read the Tucker Max forums from time to time as well, some very smart and interesting people on there in amongst the people whose sole contribution appeared to be "you've slept with XXXX women, you're my hero...". It's not a forum you could normally have browsed at work but it was good on occasion, sadly it's defunct now although I believe that there are various offshoots still going.