VirtualBox, developed by Oracle, is cross-platform x86 virtualisation software. It has become one of the most widely used virtualisation solutions in the world.
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.
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.
OpenCart is popular, powerful, and easy to use shopping cart software. Their healthy community forums are powered by phpBB.
The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.
Conquer Club is an online multiplayer world domination game. The discussion forum is host to over 500,000 members and 2.8 million posts.