Official Support Forum for Debian, the kernel-based open source operating system. phpBB provides a solid platform for support and discussion.
Solia Online is one of the most active online communities with over 18 million posts. Solia Online allows you to create your own avatar, buy items, and discuss topics with your friends.
Pixelmator is a popular commercial image editing software for Mac OS X users.
As the official board for the world-famous band Queen, this community hosts a wide variety of discussions by Queen fans from around the globe.
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.
KDE is mainly known for producing the KDE Desktop Environment (used in Kubuntu) for Linux.