Joomla is one of the leading open-source Content Management Systems. With a userbase of over 400,000, they use phpBB as a reliable platform for support.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
Info-Mac is a community dating back to 1984 as a mailing list and was the first online community for Apple's Macintosh computer. All of their content was converted to phpBB in 2007.
Conquer Club is an online multiplayer world domination game. The discussion forum is host to over 500,000 members and 2.8 million posts.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
OpenCart is popular, powerful, and easy to use shopping cart software. Their healthy community forums are powered by phpBB.