Info-Mac
Info-Mac is a community dating back to 1984 as a mailing list and was the first online community for Apple's Macintosh computer. All of their content was converted to phpBB in 2007.
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.
OpenOffice.org
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.
YUI Library
Emerging from the Yahoo! Developer Network, YUI has become one of the more popular open-source JavaScript libraries.
VideoLAN
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.