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.
Professional online broker and trading community.
Developed by Focus Home Interactive, Cities XL 2011 is a city simulation game. Their community forums are powered by phpBB.
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.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.