phpBB2 is a free, widely used, message board that is highly customizable. With it you can set up a message board with moderators and admins. The moderators can delete and edit people's posts in the forums they are assigned too and the admins can do anything from making a copy of the database to creating new forums. In the next paragraph I will go over a couple of my favorite features in the forum, but I am not going to list all of them. This is because, first of all there are tons of features, and second of all you can get lots of "hacks" (mods, edits, changes, or whatever other words you use) to add even more features to your board.
One of the first cool features (other than being free [Open Source]) is that you can have as many forums or categories as you want on your message board. Another cool feature is the rankings you can assign. This is where you can have a little "status message" like thing under a person's name, depending on how many posts they have made. Finally, the coolest thing about this phpBB2 is that you can customize it to blend in with your site seamlessly. Of course learning to customize it will take some time as for you will have to read some tutorials and be good at web design.
There are 2 other things I would like to say before I make my conclusion. First of all I would like to say that this isn't very hard to set up at all. Basically all your have to do is enable a MYSQL database on your server, upload the files to your server, and run the set-up and if you can't get it working for some reason I will be more than happy to help you or maybe even install it for you if I have time. Just E-Mail me at email@example.com and I will see what I can do:-) Second of all I would like to tell you all about this site which offers a whole bunch of hacks for your phpBB2 Board. It is http://www.phpbbhacks.com/ .
So in conclusion, this is probably one of the best (yes, there are some other ones you can find) free Message Boards there is. Also if you want to see how it works you can just check out my own Message Board HERE or go to your favorite Message Board and chances are it uses phpBB2 Board.