We have implemented phpBB in response to several requests that there be a threaded discussion forum. The Elgg is great for making blog statements and commenting on posts, but for extended discussion with complex turns it is not a easy as a forum. Similarly, while the Moodle forums are popular they are, for the most part, linked to time-bounded events and the topic structure of Moodle makes inter-topic threads dificult to follow.
So, here is a forum and nothing but a forum.
Please let me know what you think.