In this tutorial from Matthew Leverton, there's information on adding a step several phpBB users could benefit from - putting a CAPTCHA image on the comment form to reduce spam issues.
phpBB is "an open source, flat style, message board written in PHP." Because thousands of people use it with little or no modifications, it is a very popular target for spammers. They can use bots to automatically fill out the registration forms, including "clicking" on the activation link inside e-mails. A CAPTCHA is some sort of challenge that will (in theory) be easy for a human to solve, but hard for a computer to. Typically, they are implemented as a distorted word that the user must enter to complete the form.
He walks the reader through each step of the way, giving shell commands and configuration changes where appropriate. The addition makes use of the freecap CAPTCHA script, making it a simple process of including the library in and adding a few lines to the comment form code inside phpBB.