If you're looking for the simplest of the simple, I'm fast becoming a fan of SimpleForum (simpleforum.net). Right now I'm using it as a temporary board for my residence hall while our regular server gets fixed. But you can also see the results at
www.d5robotics.com if you'd like. I had the version I'm running up in about fifteen minutes. It would've been five if I had remembered to upload everything to the server.