http://www.harmonicdiscord.com is one of my favorites . . . as I was a member there before I realized headphones could offer better sound for cheaper. Good discussions there. I fell in love with nOrh speakers after I joined there; as many do. Everyone on that board either owns nOrh speakers because they love their price and their sound, and because its contagious.
http://www.audioasylum.com is very well known and its members are referred to as "inmates." It is a vey good message board, for people there are very into audio, considering many have systems over $20,000, many with systems upwards of $50k and $100k too!
Lets not forget this wonderful site, also best place to buy/sell audiophile gear. I prefer this to above mentioned Asylum which is also good but tends to be heavily influenced by DIY mindset.
There are members here who are true fanatics and extremely knowledgeable.