Zend Technologies is one of the largest and most popular companies producing a wide variety of tools and frameworks for PHP. Their community forums rely on phpBB.
Meristation Magazine is a Spanish-language website featuring articles on gaming. Their discussion forums hold over 34 million posts and are powered by phpBB.
Hugely popular Mozilla community providing support for Firefox, Thunderbird, Seamonkey and other well known Mozilla products. Over 350k members and 4 Million posts.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
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.
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.