phpBB: Free and Open Source Forum Software
Get it at:
Why you’d choose it:
- Dedicated to the concept of user forums.
- Divide up boards by topic and by permissions
- Detailed user and group management
What it doesn’t do:
Anything but bulletin boards! (This may be a plus, depending on your point of view.)
Installation:
https://www.phpbb.com/support/documentation/3.0/?from=submenu
Examples:
http://flysandia.org/phpBB3/index.php?sid=3ff47c833f31c3b206e8803f2cc9723c