Simple Machines Forum

“Simple Machines Forum — SMF in short — is a free, professional grade software package that allows you to set up your own online community within minutes. Its powerful custom made template engine puts you in full control of the lay-out of your message board and with our unique SSI - or Server Side Includes - function you can let your forum and your website interact with each other. SMF is written in the popular language PHP and uses a MySQL database. It 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. SMF is the next generation of forum software - and best of all it is and will always remain completely free! ”

http://www.simplemachines.org