LX Forums - LX platform cars, Chargers, Magnums 300s and Challengers ( LC platform but so close no one cares)
Challenger Talk - new gen Dodge Challenger forum
thirdgen.org - all centered on third gen gm f body cars
I guess I should represent. Subaru WRX STi.
I guess I'm obligated to plug my favorite one in here:
It's mainly for the Mazda6, but we've got some Miatas, some 3s, an Accord, and various other cars on there. When they say community, they mean it. It's not just a forum to talk about cars...it's like a second family. Most of the members know each other and have formed friendships. Some have met in real life, and some haven't, but with some of these people, you feel like you have known them your whole life.
priuschat.com -- Pretty obvious
clublexus.com -- All Lexi (Lexuses)
civicforums.com -- All Honda Civic generations. (Was for 7th gen, but then grew to all later)
naioa - north american impala owners association