There are plenty of open source forums available now a days. Like PhpBB, Vanilla1, YaBB, MiniBB but for all those forums, you will have to have MySQL, PHP and other resources that you need to host a Forum site. But if you want a simple one forum for your product or service, you can use this embeddable forums. Just create and embed it into any of your web page (just like embedding a youtube video to your page) and you are good to go.