PHPBuilder.com has posted a comparison of three PHP forums (the software to run them) - vBulletin, phpBB and Kunena.

In the past few years, there has been a great increase in the number of websites with user-generated content (UGC). One of the most common Web applications with UGC is a Web forum -- a place where people can share their impressions and opinions on different topics. As forum software is becoming more and more popular, I will compare the three best forum software products written in PHP: vBulletin, phpBB and Kunena.

They go through each of them and talk about what features they offer and some of the pros and cons of each. At the end of the post there's a table laying out each of these in a bit more easy to reference form.