Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
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.
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 Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.
Pixelmator is a popular commercial image editing software for Mac OS X users.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.