KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
Pixelmator is a popular commercial image editing software for Mac OS X users.
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.
Discussion forum for the massively popular American animated sitcom. Powered by phpBB.
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.
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.