Seriously, canadafred has a legitimate question. I suspect the answer to your own question will come if you stop and really think exactly what type of person you would like to go to your forum, what your forum is all about, and what those type of people want in a forum? Why should they go to your forum instead of the millions of others out there?
So does anyone know of good free traffic that's useful? I'ver never heard of any. Oh well.
... other than the free targeted traffic that is available in the natural search results. That's pretty useful traffic. Those are the ones that can make the telephone ring and send the email enquiries. That's pretty useful traffic. That could be the kind of traffic that a web site would want, I should think.
Imagine people doing searches and finding a web site and an interested web surfer clicks through the link to a web site. Can you comprehend the concept? The visitor actually wants to go there. He visits the web site because he wants to. He naturally wants to visit the web site. I know this could be a tough one to get for many people, but it happens in the natural results from time to time. That is why I am saying it three different ways. There is life beyond paid advertising. Wake up. It is time to sharpen your pencil.