This is the best message board I've been on.
I've visited this one that had a science fiction flavor to it. Nightly.net - It's pretty good, but they talk about Star Wars and Star Trek and Battlestar Galactica and a whole bunch of other stuff. It's a pretty busy board, with an Open forum, as well (I think it has an open forum).
My local newspaper has a very active message board, too.
www.indystar.com There are a lot of "trolls" on that board, though. Lots of hating, name-calling, and insulting going on. It gets very ugly there. I used to post there all the time, but I got sick of the same stuff, different day. I rarely post there anymore. The indystar board taught me how to have a thick skin on a message board, though. You say what you need to say, be honest to who you are but respectful at the same time, don't take too much too seriously, and move on.
I've visited another one for a local periodical called Nuvo Weekly. It's about as bad as the indystar board, but it's more "open," with the mods allowing profanity to a certain extent.
I guess you could say I'm an experienced message board poster.