KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.
Conquer Club is an online multiplayer world domination game. The discussion forum is host to over 500,000 members and 2.8 million posts.
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.
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.
The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.
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.