Are you looking for a breed specific one, a holistic one, or just a general dog forum with a bit of everything? I know of several really good ones i go to often. PM if you want the links, i'm not sure if i'm allowed to post them here.
lab-retriever.net is a great one. that is where i am most of the time when i'm not on MDC. they have lots of people that are into natural pet care and nutrition and you can get greast advice. there is even a holistic vet that posts there ocassionally.