xkcd
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.
Ubuntu Italy
It is the Italian Community for Ubuntu, a popular open source community operating system based upon the the Debian Linux Distribution.
Debian
Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
The Audi TT Forum
Active community for UK based Audi TT owners. Almost 30k members and 2 million posts, complete with unique style & powered by phpBB.
OpenOffice.org
OpenOffice.org offers a free and fully-featured office suite which meets the needs of the common consumer all the way up to large enterprises.
Info-Mac
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.