PNUO

If you’d like to add forums to your WordPress blog, you have a number of options available to you. Here are the three I have experience with and recommend as viable options:

1. BBPress – This is a free and lightweight open source forum software that was designed specifically for WordPress users. It is also currently owned and supported by the WordPress team (Automattic). In order to use this software, you simply need to download the software, place it on your server where you want your new forums to be, then go to that website and use the installer to finish getting setup. It comes with an admin panel so you can customize your forums to look and work the way you want them to.
2. PhpBB – PhpBB is another free open source forum software that is known for its customizability. The newest version, version 3.0, offers a great forum look for your website. PhpBB also has a skins folder (themes) and many premium WordPress theme authors offer free forum skins if you buy their themes, including Brian of Revolution themes. If you go this route, you can simply upload the skin and activate it within your forums control panel to make your PhpBB forums match the look of your premium theme!
3. VBulletin – This is by far your best option, and probably the forum software you see being used on most of the popular forums you visit regularly. Unfortunately, though, you will have to purchase a license to use this software. At last check, you can either lease your license or own a license. Leasing is roughly $100.00 a year, but you can own the software for $180.00. They also offer support for an additional fee.

If you aren’t comfortable doing things like running an installer and setting up your own MySQL database, there are also a few WordPress plugins that claim to add forums to your blog. I haven’t used them personally, but figured they deserve a mention here so you know all of your options:

• RS Discuss
• XDForum