esoTalk is a free, open-source web-forum application with a bunch of cool features.
It's designed to be super-simple, super-fast, lightweight, and extensible.

NinkoBB is an open source forum script written in the PHP language and uses a MySQL Database. NinkoBB is designed to be as simple as possible and provide you with the key features that you need, all the while keeping the space used on your server to a minimum.