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.
Pixelmator is a popular commercial image editing software for Mac OS X users.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
Adblock Plus is one of the most popular Firefox Add-ons in the world, with over 10 Million daily users and over 100k downloads a day. They use phpBB to offer support to their users.
Professional online broker and trading community.
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.