For many websites, a user forum is a necessity. For non-commercial sites, they provide a way for users to communicate with one another, and for many commercial sites, they are a good way to provide customer service.
I recently had to choose one for my newest website. Since the hosting server is Unix-based and I've become a big PHP fan, I looked only for BBSes written in PHP. I found five BBSes written in PHP. Three are open source. The other two are modestly priced. All of them make use of MySQL for storage.
Phorum (www.phorum.org)I actually have some experience with this particular BBS. I've installed it on two different websites. Installation is quick and relatively painless, and it's easy to customize, too.
I added a feature to the board which allows users to subscribe to the board by e-mail. Users simply type their e-mail address into a form, and a script appends that address to a list of e-mail addresses. When a user posts to the board, the script that enters the post into the database also emails it to the list of e-mail subscribers.
I've also written a module for phpWebSite that queries the Phorum database and displays a list of Phorum topics on phpWS pages.
PHPbb (phpbb.com)This one seems to be the favorite of the PHPNuke crowd. I don't have any experience with this one, but in surfing around some of the sites that use it, I must say that I like the way it looks. One thing I like is that there's a website devoted solely to hacks for PHPbb- phpbbhacks.com.
I recently tried installing this on one of my sites, but because PHP is running as a CGI program there rather than an Apache module, it wouldn't run right. I had to use Phorum after all.
XMBForum (www.xmbforum.com) I don't like the item layout much, but this board has a feature that the others lack: the ability to attach files, including graphics. You can, for example, upload a graphic which is your signature and appears on every post you make. You can also mark up your posts with html-like tags to get certain formatting effects and send private messages to other users. I don't know if it's included with the download, but it has a nice FAQ.
UBBThreads (www.infopop.com) This software costs about $250 for the PHP version. I was hired to install and customize this software for a client. They liked it better than Phorum, even though they had to pay for it.
The company planned on using the board for customer support, and they wanted to require that users register and be approved before they were given a login. This was relatively easy to do, as the coding of the BBS was very professional and almost self-explanatory.
This software also has a more finished look to it, but I'm not sure that characteristic was worth paying for. A Perl version is also available.
vBulletin (www.vbulletin.com)I really know nothing about this software. I just found it by accident while reading another forum. It's not free, but like UBBThreads it is not very expensive.