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.
The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.
OpenCart is popular, powerful, and easy to use shopping cart software. Their healthy community forums are powered by phpBB.
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.
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.
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.