I guess a big one is Avsim (http://forum.avsim.net/). Funny mixture of folks, some are  , some others are   and me is  . Well, that's a lie, I'm actually   all the time, but I cover that up. From the amount of information around, I guess that one is a good tip.

Others are http://forum.simflight.com/ and http://www.flightsim.com/, but there are more and each of them may state that they are #1.  

CS forums have the highest amount of nuts people, which includes me.