phpbb

This is a module that lets Drupal handle user- and session-management for phpBB, a popular forum / bulletin board system. In practice it takes things from the Drupal database and updates the phpBB database each time a user logs in. It also provides a block that lists recent topics from the forum. This module requires mod_rewrite / clean URLs and is not recommended for beginners.

The installation instructions can be found here. If you have questions or other concerns you should discuss those over at http://groups.drupal.org/phpbb, don't forget to say what versions of phpBB and Drupal you are using.

Demonstration sites:
Drupal 5.5 and phpBB 3.0 (instructions)
Drupal 5.5 and phpBB 2.x (instructions)
Drupal 4.7 and phpBB 2.x (instructions)

Note that these are just very simple demo sites, if you want to see large sites in production (albeit in norwegian) you should have a look at http://mac1.no and http://linux1.no

Note that the modules for phpBB 2.x and 3.x are rather different, but the matrix of workable combinations is as follows:
Drupal 6.x :: phpBB 3.x (planned)
Drupal 5.x :: phpBB 2.x and phpBB 3.x
Drupal 4.x :: phpBB 2.x