Ars Technica
Boasting almost 14 million posts, Ars Technica is one of the biggest and oldest technical forums still operating today. They use phpBB to power their forums and comment system.
Meristation
Meristation Magazine is a Spanish-language website featuring articles on gaming. Their discussion forums hold over 34 million posts and are powered by phpBB.
Avant Browser
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.
YUI Library
Emerging from the Yahoo! Developer Network, YUI has become one of the more popular open-source JavaScript libraries.
Adblock Plus
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.
Gentoo
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.