OpenCart is popular, powerful, and easy to use shopping cart software. Their healthy community forums are powered by phpBB.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
Inspired by Max Berry's book, Jennifer Government, NationStates is one of the most popular online simulation games. The forum boasts over 5 million posts and 200,000 users.
Discussion board for the massively popular xkcd web comic. Covering everything from Maths and Science to Logic Puzzles and Forum Games. Over 100k members and 2.4 Million posts.
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.
Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.