I read and post on:
ATV connection forum
kawieriders.com

There is also the following forum that I go to as well.
Mud and Dirt.com
ATV Nation.com
TexasATV's.com
ATV Mud Maina.com
Mud Throwers.com
Utility ATV Riders.com
Killer ATV.com
ATV Frontier.com

There are a few more, but these should get you started. Personally, I think the best are the atv connection forum and kawieriders.com (if you have a kawasaki). The atv connection forum is probably the biggest.

Good luck.