drupal php2drupal module
http://drupal.org/project/phpbb2drupal 6.x-1.4 on 6.2 (if I remember correctly)
Main function:
The phpbb2drupal module imports phpbb forum database data into drupal and makes it functional as a drupal forum.
Example function:
Its main use is for migrating a phpbb2 forum into a drupal forum after you have decided that you want to switch to drupal and take your previous phpbb forums with you.
The module is very well documented. The instructions are very, very good.
Experience:
As an exception, this is written a good 9 months after doing the migration. I recall that you need to be careful that existing drupal usernames are not going to conflict with usernames that will be imported from the phpbb forum.
I also recall that while the import goes fine, the above issue and possible other small ones that I don't recall now, prompted me to do the conversion twice or three times, until it turned out the way that I wanted it. Making a full, functional backup and securely knowing (practice) how to re-install drupal to a previous state is essential.
Next steps:
After the successful install, you'll be confronted with the question of how to control user access. You'll also want some features like susbcriptions to forums, the ability to send private messages, and so on. These are all not included in drupal core (or the core forum module).
For access control, I initially tried tac_lite, but it wasn'ty adequate, and I switched to forum access instead - not sure why I did not use it in the first place! The others I'll elaborate later.