Message Boards (forums or guest books) are great for encouragiong your users to return to your website. They are esential for establishing a sense of community amongst your visitors. A well run, popular forum can be the difference between a dud website and a great website. The following forums listed here are either free, low cost or unique in some way. They will either be complete packages that you can install yourself and host on your own server, or they can be shared resources, hosted on another server, where you just add a reference to it on your page.
A very nice looking and professional service. Giving you free, remote hosted message boards for your website.
This gets my vote as the best forum software around. If you can manage your own server and run ASP (Active Server Pages) this would probably be one of the best to use. Extensive features and a great support forum run by users, I use it to run the Sticky Forums Section of this website.