phpBB2

A widely-popular open-source bulletin-board package, works well, simple user interface and admin panel, clean look, scales well, and can be customized.

SMF

Simple Machines Forum or SMF is a free, professional grade Internet forum script that allows the setting up of your own online community within minutes. Written in PHP and MySQL backend database, Simple Machines Forum is designed to provide you with all the features you need from a bulletin board while having an absolute minimal impact on the resources of the server.