i used to be in off-topic.net but they keep deleting my account for inactivity, otherwise:
board.hondasociety.com
www.superhonda.com
www.honda-acura.net
www.audiosiege.com
www.nissanforums.com -> i've only been to DNE section
www.dfwstangs.net -> for sale forum mostly
www.honda-tech.com
www.elitecaraudio.com -> for sale forum mostly
www.the-lot.net