At first my intelligence was insulted [that phpBB made the list of Best Open Source Websites], but then I saw the subtitle of the article:

Ana Kronschnabl and Tomas Rawlings pick the best open source websites where users can change the content

and then, while obviously not in the way the authors had intended it, it made perfect sense.

Some of the other sites/software (non-PHP) makes up for its inclusion in the list, including: Wikipedia, Firefox, GIMP, and SourceForge...