I’m using phpBB on the Christian Debate forum. Actually, I had wanted to originally use Invision. But, being the cheapo that I am, I didn’t want to pay any money for a forum. It turns out though that phpBB is pretty good. What I like about it is that there are a lot of official mods for it. And I was able to install several of them with no problems at all. You do have to be a programmer to understand how to do the mods, but anybody who knows basic PHP can do it. Overall, I’ve been satisfied with phpBB, especially considering I’ve spent nothing for it.