I'll contribute, since I know of a couple.
http://www.mini2.com -- Very nice community dedicated to Mini2fosi.
http://www.lancisti.net -- Lancia fans.
http://www.vwvortex.com -- Besides posting on VAG, they also do fansites for
Subaru, Volvo, and everyone's favorite Pontiac, the
Vibe.
http://www.vtec.net -- For Honda / Acura fans ... has a very nice
model release matrix for just about every new Honda to come out in the next 5 years or so.
http://www.saturnfans.com -- Pretty nice Saturn forums, tho' I'll say that Saturn forums look an awful lot like Apple forums. That whole "we love you, but PLEASE make some competitive products" kinda thing.
http://forum.granturismo.com -- Believe it or not, a group of forums about a GAME, and the young demographic that comes with it, actually attracts a group of SERIOUS car buffs and motorsport fans. Lots of good posts in The Open Road and Motorsports, one of the mods (Taimar2) is especially well versed in automotive history, most of his posts are fantastic.
I'll come up with more when I remember em.