When I look into Open source software, it usually means free software to me. Many of the message boards and CMS' that we use today are in fact open source.

An example of one open source software today would be Joomla. It is a content Management System. Joomla is a derivative or fork of Mambo. With open source you are allowed to make derivatives of another software as long as you submit it under the same license as the original. Simple Machine Forums (SMF) used to make plugins for Joomla but stopped due the license terms.

Take a look at the Open Source Initiative for what you can and cannot do. They are also a good example of a site using open source software as they are using Drupal as their CMS.

Forums

• Phpbb
• MyBB
• SMF

CMS

• E107
• Drupal
• Joomla
• Mambo

There is many more open software and it can be located at http://www.opensourcecms.com