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.
Avant Browser is a fast and user friendly browser which uses Microsoft's Trident rendering engine. The browser has been downloaded over 22 million times.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
Pixelmator is a popular commercial image editing software for Mac OS X users.
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.
As the official board for the world-famous band Queen, this community hosts a wide variety of discussions by Queen fans from around the globe.