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.
Discussion forum for the massively popular American animated sitcom. Powered by phpBB.
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.
GameServers is a premier Game Hosting company, having collaborations with big names such as Infinity Ward, Treyarch and Epic Studios.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.