When you need to add Forum to your WordPress blog, our first option is to use phpBB or VBulletin if you have some dough to spare. However, this method is not recommended for blogs that are already online and you want to integrate forums to it.
WpHacks.com suggest alternatives into getting forums to your WordPress blog that can be added with just a few simple steps using RS Discuss WordPress plugin. Expect to get the functionality of a forum like the ability to set administrators and moderators, multiple language support, and RSS feeds.
If you need a quick forum to your WordPress blog, this plugin is the right one for you.
Read full article here: http://wphacks.com/how-to-easily-add-a-forum-to-your-wordpress-blog/