The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums.
A very active community for a KFAN-AM, a popular sports radio station in Minneapolis/St. Paul, MN.
12 Chat Forums is one of the largest and most active social and chat communities in the Arabic language. Their community boasts nearly 2 million posts and 80,000 users.
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.
Gentoo Linux is an open source operating system built on top of the Linux kernel and based on the Portage package management system.
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.