Phpbb is a very powerful forum solution, and with millions of downloads worldwide, it has quickly become the most widely used open source forum in the world. Phpbb runs on a powerful PHP core and MYSQL databse. The software has been downloaded millions of times, and has advanced features such as multi-database support for a long term forum solution.
SMF - FREE!
SMF is a well established PHP bulletin board. It is loaded with powerful features, but at the same time, is very efficient with server resources. SMF classifies them selves as being the next-generation forum software. And yes, it’s free.
Advanced Electron Forum - FREE!
AEF (Advanced Electron Forum) is a very small, lightweight, but powerful forum script. Weighing in at just under 1mb, it's significantly smaller than it's competitors. AEF has a very easy installation procedure, and a simple admin panel. AEF also has a few other features that even the big boys don't have, such as the ability for users to re-arrange forum categories for themselves.
