I have an audi a4 , so I'm on Audiworld.com on the A4 Forum, the forums move pretty quickly..
I'd have to say that
www.vwvortex.com has to be up there.
The car forums are definately high moves.
www.mbworld.org
www.clublexus.com
www.streetkias.com <-- feel free to go on there and harass the kia owners, many of us on the A4 / audiworld forums have accounts on there, we're trying to get some races going :-)
howardforums.com for cell phones
www.dealdatabase.com - tivo/dtivo hacking forum
audizine.com and socaleuro.com are owned/run by some people I know, ones in San Diego, and ones in Norcal.
Craigslist.org moves pretty quickly, but thats not really a forum huh?