Hugely popular Mozilla community providing support for Firefox, Thunderbird, Seamonkey and other well known Mozilla products. Over 350k members and 4 Million posts.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
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.
Active community for UK based Audi TT owners. Almost 30k members and 2 million posts, complete with unique style & powered by phpBB.
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.
The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.