The Jehovah's Witnesses have a HUGE Watchtower Kingdom Hall in Brooklyn (which I think may still be their world headquarters), and other smaller facilities.

I don't really care what they believe, but I got fed up with them waking me up on the weekends by ringing my door buzzer. Some years ago, I had a small sign made up that said "NO JEHOVAH'S WITNESSES", with the word "AWAKE" (the name of one of their publications) in the middle, encircled in red and crossed out with a line across the word. I don't have time right now, but I'll scan it soon and post it.

They are not allowed to have blood transfusions - and this is very dangerous. Some of them hold off on necessary treatment for themselves and/or their children.
http://www.watchman.org/jw/jwtransfusions2000.htm