Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
The VideoLAN project produces the extremely popular VLC media player software. Their forums provide support for users of the software as well as the development community.
Boasting almost 14 million posts, Ars Technica is one of the biggest and oldest technical forums still operating today. They use phpBB to power their forums and comment system.
Pixelmator is a popular commercial image editing software for Mac OS X users.
It is the Italian Community for Ubuntu, a popular open source community operating system based upon the the Debian Linux Distribution.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.