Rennlist for mature and knowledgeable members and very little b.s.
http://forums.rennlist.com/rennforums/index.php
Pelican Parts Technical BBS for the best tech info and no boy racer crap.
http://forums.pelicanparts.com/
Reutterwerk Forums is the new "Rennlist" (started by John D., the guy who started Rennlist and then sold it to IB) Not a lot of traffic yet, but promising.
http://reutterwerk.com/forums/