I'm not including CARM where I think you have to accept the Trinity concept to even post, but if you have a verrrry thick skin and can calmly reply to caustic insulting comments with a message that teaches a gospel principle, you can eventually make some inroads on boards where you'd expect most members to be quite anti-LDS.
I started an "Ask a Mormon" thread on such a site a couple of months ago and today it has had more than 10,000 views, far more than any other thread their religion forum has ever generated.
There's a whole lot of interest in what Mormons believe. Harsh mocking opposition generates break out the popcorn interest, and opportunities to calmly teach gospel principles and promote
Mormon.org to the lurkers.
As my Ask a Mormon thread progressed I came to know that most of the critics were former or inactive members of the LDS Church!
One thing to keep in mind is that you don't have to play within the boundaries of someone else's sandbox. If responders try to politicize the discussion for example and make it a conservative vs liberal topic, you can ignore that and calmly reply with posts that bring it back to God and our relationship with him. (Especially if it is a "Religion" forum.)
I think I'm doing some good as an LDS Cyber Missionary even in the darker corners of CyberSpace. I'm sure others can do the same.
It's not for everyone, it requires thick skin, a calm temperament, and an unshakeable testimony. If you have those qualities, why not start an "Ask a Mormon" thread somewhere today?
You can learn more about LDS Cyber Missionary work at
http://www.lds1.org