I'm a member here obviously, and on the SAE forum, though I don't participate there much. The format of the forum is a little odd.
Likewise with the Trains forum, a member there, but it too is odd.
I participate on Nscale.net quite frequently, as I'm also interested in model railroading.
67-72chevytrucks.com is another forum which I am am member of, but don't actively participate in. Mostly just browse and look at pictures.
I recently also joined abody.net and the chevycitationsforever yahoo group as well, but mostly to peruse and look at pictures.
What other online communities do you belong to, model car or otherwise?