Hi Craig,AudioRoundTable.com is powered by a modified form of WWWBoard. It's the original WWW messageboard, and it has some really attractive features. There are others too, each with their own strengths and weaknesses. Examples include vBulletin, phpBB and Colloquius.
All of your options are actually pretty inexpensive to setup. What costs the most is development, should you require added functionality. Maintenance can become expensive when the site is popular, even if only moderately. But the initial investment is small, often times free, and sometimes tied to an annual service agreement with your ISP.
When AudioRoundTable.com was being formed, I donated a lot of time developing features that I always wanted over at the old forum, and eliminating some of the chaffe that I never really liked or thought was needed. I've decided to make this Parham Data version of WWWBoard available, and we can discuss it off-forum if you're interested.
Wayne